Collisional Energy Loss in a Finite Size QCD Matter
解决自然资源过度消耗问题的英语作文
In the face of escalating global environmental challenges, the overconsumption of natural resources stands as a critical issue that requires immediate and comprehensive attention. The relentless depletion of our planet's finite reserves, exacerbated by population growth, industrialization, and unsustainable consumption patterns, not only threatens biodiversity and ecosystems but also jeopardizes human well-being and prospects for future generations. This essay presents a multifaceted approach to tackle this complex problem, encompassing technological innovation, policy interventions, societal transformation, and international cooperation.I. Technological Innovation: A Catalyst for Resource EfficiencyTechnological advancements play a pivotal role in mitigating the overconsumption of natural resources by enhancing efficiency, promoting circular economies, and fostering the transition to renewable energy sources.A. Resource-Efficient Technologies: Innovations in manufacturing processes, product design, and infrastructure can significantly reduce resource consumption. For instance, industrial symbiosis models, where waste from one industry becomes input for another, minimize waste generation and promote resource recycling. Moreover, digital technologies like the Internet of Things (IoT) and artificial intelligence (AI) enable precision agriculture, reducing water and fertilizer use while increasing crop yields. In the built environment, green building design and smart energy management systems optimize energy and material usage, contributing to substantial resource savings.B. Circular Economy: Embracing a circular economy model, where materials are kept in use for as long as possible, is crucial for decoupling economic growth from resource consumption. This involves designing products for durability, repairability, and recyclability, implementing effective waste management systems, and fostering a market for secondary raw materials. Breakthroughs in material science, such as the development of biodegradable plastics and novel composites, further facilitate the transition to a more sustainable material cycle.C. Renewable Energy Transition: Shifting from fossil fuels to renewable energy sources like solar, wind, hydro, and geothermal power is essential for reducing the reliance on non-renewable resources and mitigating greenhouse gas emissions. Technological advancements in energy storage, grid integration, and distributed energy systems have made renewables increasingly competitive and scalable, paving the way for a low-carbon energy future.II. Policy Interventions: Steering Societies towards Sustainable ConsumptionEffective policies are indispensable in steering societies towards more sustainable consumption patterns, incentivizing eco-innovation, and ensuring the equitable distribution of resource benefits.A. Economic Instruments: Implementing economic instruments such as carbon pricing, taxes on resource extraction or consumption, and subsidies for environmentally friendly alternatives can internalize the costs of resource depletion and incentivize more efficient resource use. Additionally, tradable permits or cap-and-trade systems can create market incentives for reducing resource consumption and emissions.B. Regulatory Frameworks: Strengthening regulatory frameworks to enforce resource efficiency standards, ban environmentally harmful practices, and promote circular economy principles is vital. This includes establishing extended producer responsibility (EPR) schemes, mandating eco-design requirements, and enforcing strict waste management regulations.C. Education and Awareness: Public awareness campaigns, educational programs, and consumer labeling initiatives can empower individuals to make informed choices, foster a culture of responsible consumption, and drive demand for sustainable products and services.III. Societal Transformation: Changing Mindsets and BehaviorsAddressing overconsumption necessitates profound changes in societal values, lifestyles, and consumption habits, which can be facilitated through education, cultural shifts, and community engagement.A. Education for Sustainability: Incorporating sustainability education into formal curricula and lifelong learning initiatives can nurture a generation of environmentally literate citizens who understand the implications of resource overconsumption and are equipped to adopt sustainable lifestyles.B. Cultural Shifts: Encouraging a shift away from the prevalent 'throwaway culture' and promoting values of sufficiency, sharing, and collaboration can help reduce excessive consumption. This involves fostering a re-evaluation of the concept of prosperity, moving beyond the narrow focus on material wealth to encompass well-being, social connectedness, and ecological harmony.C. Community Engagement: Empowering communities through participatory decision-making, local initiatives, and grassroots movements can stimulate bottom-up solutions for resource conservation and sustainable living. Community-based projects, such as urban gardening, shared mobility schemes, and repair cafes, not only conserve resources but also foster social cohesion and resilience.IV. International Cooperation: Tackling a Global ChallengeOverconsumption of natural resources is a global challenge that necessitates collective action and international cooperation.A. Multilateral Agreements: Strengthening and expanding multilateral environmental agreements, such as the Paris Agreement and the Convention on Biological Diversity, can provide a framework for coordinating global efforts to reduce resource consumption, mitigate climate change, and protect biodiversity.B. Knowledge Sharing and Capacity Building: Facilitating knowledge exchange, technology transfer, and capacity building between nations can accelerate the adoption of resource-efficient technologies and practices, particularly in developing countries where resource consumption is rapidly increasing.C. Financing Mechanisms: Mobilizing financial resources from public, private, and philanthropic sectors to support resource conservation initiatives, especially in underprivileged regions, is crucial. Innovative financingmechanisms like green bonds, impact investing, and results-based financing can channel investments towards sustainable projects.In conclusion, addressing the overconsumption of natural resources requires a holistic, multi-stakeholder approach that harnesses the power of technological innovation, policy interventions, societal transformation, and international cooperation. By embracing these strategies, we can chart a path towards a more sustainable future where the Earth's finite resources are managed responsibly, ensuring the well-being of current and future generations while preserving the integrity of our planet's ecosystems.。
环保绿色行动的英语作文
Environmental protection and green action are crucial for the sustainable development of our planet.Heres an essay on the importance of these initiatives and how we can contribute to them.Title:Embracing Green Actions for a Sustainable FutureIn the modern era,the concept of environmental protection has become increasingly significant as we grapple with the consequences of industrialization and urbanization. The degradation of natural habitats,pollution,and climate change are pressing issues that demand immediate attention and action.As global citizens,it is our collective responsibility to embrace green actions that not only mitigate these problems but also pave the way for a sustainable future.The Importance of Environmental ProtectionEnvironmental protection is essential for preserving the earths ecosystems,which are the foundation of life.It ensures the conservation of biodiversity,which is vital for maintaining the balance of nature.Moreover,it helps in reducing pollution levels,which directly impacts human health and wellbeing.By protecting the environment,we are safeguarding our water sources,air quality,and soil fertility,which are critical for agriculture and overall survival.Green Actions:A Call to ActionGreen actions encompass a wide range of activities that promote sustainability and minimize the negative impact on the environment.Here are some key areas where we can make a difference:1.Reducing,Reusing,and Recycling:This principle,often abbreviated as the Three Rs, is a fundamental approach to waste management.By reducing our consumption,reusing items,and recycling waste materials,we can significantly decrease the amount of waste that ends up in landfills.2.Conserving Energy:Energy conservation is not only about saving money but also about reducing our carbon footprint.Simple actions like turning off lights when not in use,using energyefficient appliances,and insulating homes can make a big difference.3.Sustainable Transportation:Opting for public transport,cycling,walking,or carpooling can reduce our reliance on fossil fuels and lower greenhouse gas emissions.4.Supporting Renewable Energy:Encouraging the use of renewable energy sources like solar,wind,and hydroelectric power can help us transition away from nonrenewable resources and reduce our environmental impact.5.Ecofriendly Products:Choosing products made from sustainable materials or those that have a lower environmental impact can support companies that prioritize green practices.6.Planting Trees:Afforestation efforts can help combat deforestation and climate change by absorbing carbon dioxide and providing habitats for wildlife.cating and Raising Awareness:Knowledge is power,and educating ourselves and others about the importance of environmental protection can inspire more people to take action.The Role of Governments and OrganizationsGovernments and organizations play a pivotal role in implementing policies and initiatives that promote environmental protection.They can legislate against pollution, support research into sustainable technologies,and provide incentives for businesses to adopt green practices.ConclusionIn conclusion,embracing green actions is not just a choice but a necessity for our survival. It requires a collective effort from individuals,communities,governments,and international bodies.By taking small steps in our daily lives and supporting larger initiatives,we can contribute to a healthier planet and ensure a sustainable future for generations to come.Let us all commit to being part of the solution rather than part of the problem.。
关于清洁能源的外语名称
关于清洁能源的外语名称Clean Energy: Shaping a Sustainable FutureIntroductionClean energy, also known as renewable energy, refers to energy sources that are replenishable and have minimal impact on the environment. As the world faces the challenges of climate change and the depletion of finite fossil fuel resources, the transition to clean energy has become imperative. This article explores the significance of clean energy and the various sources and technologies associated with it.The Importance of Clean EnergyClean energy plays a crucial role in mitigating climate change and reducing greenhouse gas emissions. Unlike fossil fuels, which release harmful pollutants when burned, clean energy sources such as solar, wind, hydro, and geothermal power generate electricity without emitting greenhouse gases. This not only helps combat global warming but also improves air quality and public health.Renewable Energy Sources1. Solar Energy: Solar energy harnesses the power of the sun to generate electricity. Photovoltaic (PV) cells convert sunlight directly into electricity, while solar thermal systems use the sun's heat to produce electricity or provide hot water and space heating. Solar energy is abundant and widely accessible, making it a promising clean energy source.2. Wind Energy: Wind energy utilizes the force of the wind to generate electricity. Large wind turbines, often installed in wind farms, capture the kinetic energy of the wind and convert it into electrical energy. Wind power is an increasingly popular form of clean energy due to its scalability and cost-effectiveness.3. Hydroelectric Power: Hydroelectric power harnesses the energy of flowing water to generate electricity. Dams and reservoirs store water, which is then released to drive turbines and produce electricity. Hydroelectric power is a mature technology and accounts for a significant portion of the world's clean energy production.4. Geothermal Energy: Geothermal energy derives heat fromthe Earth's internal thermal energy. Geothermal power plants use steam or hot water reservoirs underground to generate electricity. This renewable energy source is reliable and available throughout the year, making it a viable alternative to fossil fuels.Clean Energy Technologies1. Energy Storage: Energy storage technologies are crucial for ensuring a stable and reliable supply of clean energy. Batteries, pumped hydro storage, and thermal energy storage systems allow excess energy to be stored and used when demand is high or renewable energy sources are not available. Advancements in energy storage are essential for the integration of intermittent sources like solar and wind power into the grid.2. Smart Grids: Smart grids enable the efficient management and distribution of electricity. By integrating advanced communication and control systems, smart grids optimize energy usage, balance supply and demand, and enable the integration of clean energy sources. This technology plays a vital role in maximizing the potential of clean energy andensuring a reliable power supply.3. Electric Vehicles: Electric vehicles (EVs) are an essential component of the clean energy revolution in the transportation sector. EVs run on electricity, reducing dependence on fossil fuels and lowering emissions. The widespread adoption of EVs, coupled with clean energy generation, can significantly reduce greenhouse gas emissions and combat climate change.ConclusionClean energy is a key pillar in the transition to a sustainable and low-carbon future. By harnessing the power of renewable sources such as solar, wind, hydro, and geothermal energy, we can reduce our carbon footprint and minimize environmental degradation. The development and adoption of clean energy technologies, including energy storage, smart grids, and electric vehicles, will play a vital role in realizing a clean energy revolution. As individuals, communities, and nations, it is crucial that we embrace clean energy and work towards a more sustainable and prosperous future for all.。
Light Assisted Collisional Loss in a $^{8587}$Rb Ultracold Optical Trap
APS/123-QEDLight Assisted Collisional Loss in a85/87Rb Ultracold Optical TrapAnthony R.Gorges,Nicholas S.Bingham,Michael K.DeAngelo,Mathew S.Hamilton,and Jacob L.RobertsDepartment of Physics,Colorado State University,Fort Collins,CO 80523(Dated:May 23,2008)We have studied hetero-and homonuclear excited state/ground state collisions by loading both 85Rb and 87Rb into a far offresonant trap (FORT).Because of the relatively weak confinement of the FORT,we expect the hyperfine structure of the different isotopes to play a crucial role in the collision rates.This dependence on hyperfine structure allows us to measure collisions associated with long range interatomic potentials of different structure:such as long and short ranged;or such as purely attractive,purely repulsive,or mixed attractive and repulsive.We observe significantly different loss rates for different excited state potentials.Additionally,we observe that some collisional channels’loss rates are saturated at our operating intensities (15mW/cm 2).These losses are important limitations in loading dual isotope optical traps.PACS numbers:67.85.-d,37.10.Vz,34.50.CxAtomic collisions in an ultracold gas in the presence of near-resonant laser light have been studied both experi-mentally and theoretically since the advent of laser cool-ing,with both hetero-and homonuclear collisions hav-ing been studied [1,2,3,4,5,6,7,8,9,10,11,12].These light assisted collisions are responsible for limiting the densities of atoms in magneto-optical traps (MOT)[13,14,15]and play an important role in limiting the number of atoms that can be loaded into far off-resonant traps (FORT)[16,17,18,19].In this work,we describe measurements of light assisted collisions in an ultracold gas composed of a mixture of both 85Rb and 87Rb in an optical trap.By measuring the associated loss rates,we can probe the collisions associated with heteronuclear and with homonuclear long range potentials[20].We ob-serve that the resulting collision rate is a strong func-tion of the excited state potential.Since the two iso-topes are so similar,the observed loss rates are directly related to the nature of the excited state potentials,al-lowing us to probe the dependence of the light assisted collision rate as a function of interatomic potential be-tween atoms while keeping their mass,light scattering,and optical trap temperature and depth characteristics the same.While many of these potentials are complex,some are simple with only purely attractive or purely repulsive characteristics,allowing for an easier interpre-tation of the observed collision physics.In addition to providing insight into light-assisted collision physics,un-derstanding the behavior of these collisions is useful in understanding the loading dynamics of heteronuclear op-tical traps,especially those involving two isotopes of the same atom.While there has been significant recent activity in the related area of homonuclear and heteronuclear photoassociation[21],in photoassociation the relevant in-ternuclear separations are relatively short.For the long ranged potentials studied here,photoassociation will not contribute to the overall loss.Instead a related process,light assisted collisions,is the primary loss mechanismin the overall loading dynamics.Light assisted collisions occur when an atom pair,typically in a ground state,is excited to an excited state interatomic potential (Fig.1&2).After being excited,the atom pair is accelerated along the potential curve until after an excited state life-time it emits a photon and falls back into the ground state.The photon emitted is less energetic than the one absorbed,and the difference is converted into kinetic en-ergy.If enough kinetic energy is given to the atoms,they can then leave the trap,resulting in loss.To date,the majority of the relevant experimental and theoreti-cal work has been done on ultracold atoms confined to a MOT[1].Compared to a MOT,the light assisted loss is exacerbated in a FORT where trap depths are around 100µK compared to the typical 1K trap depth of a MOT.Because of the shallower depth,loss inducing collisions are much more likely to occur at a longer internuclear separation.Since the difference in hyperfine energies be-tween the two isotopes is much greater than the energy shift due to the interatomic potential at the internuclear radii relevant for loss in the optical trap,the 85Rb and 87Rb mixture behaves as a heteronuclear mixture.To understand the collision rates due to light assisted collisions as a function of potential,the long range in-teraction potentials for different combinations of collid-ing pair excited and ground states were calculated[22].To calculate these potentials,it was assumed that the interatomic distance between the two atoms was large enough so that exchange interactions could be ignored.Including the hyperfine structure,the dipole-dipole in-teractions were calculated.The large number of hyper-fine and magnetic sublevel combinations give rise to nu-merous individual interatomic potentials.Fig.1shows that there are many different types of potentials for het-eronuclear collisions:purely repulsive (Fig.1(a)),purely attractive (Fig.1(b)),or a complex mixture of the two (Fig.1(c-d)).For transitions with mixed potentials,there are numerous avoided crossings and so some initially at-tractive potentials become repulsive and vice versa.Like-a r X i v :0805.3708v 1 [p h y s i c s .a t o m -p h ] 23 M a y 2008FIG.1:Excited state potentials for heteronuclear light as-sisted loss.The zero of the scale is arbitrary.Transitions are accessed from initial ground hyperfine states of:(a)85Rb F=3,87Rb F=2,(b)85Rb F=3,87Rb F=1,(c)85Rb F=3, 87Rb F=2,and(d)85Rb F=2,87Rb F=2.Both(a)and(b)are accessed with aflash light which is detuned by60MHz to the red of the85Rb cycling transition,where as(c)and(d) are accessed with aflash light which is detuned by72MHz to the red of the87Rb cycling transition.The horizontal red lines depict laser frequency used to access each transition.wise,the excited state potentials for homonuclear colli-sions were calculated in the same manner(Fig.2).Unlike heteronuclear collisions,there are no purely attractive or repulsive excited state potentials in homonuclear colli-sions.The isotopic difference in hyperfine structure pro-duces different homonuclear excited potentials for85Rb and87Rb,but note that at the highest energy levels the structure of the potentials are qualitatively the same.In addition,homonuclear collisions are longer ranged than heteronuclear collisions.We can choose which individual potential is excited in our experiments,and can thus sys-tematically study loss rates associated with each of the potentials shown in Fig.1&2.To measure these light assisted collisions,we loaded a FORT with either one or simultaneously both isotopes of Rb.Simultaneous loading was accomplished byfirst cap-turing and cooling ultracold gases of85Rb and87Rb into their own MOTs[23].The MOTs’cooling and hyperfine repump lasers[24]for the two isotopes were aligned so that the two MOTs overlap in space.Then a30W CO2 beam was overlapped with the MOTs,and the FORT was loaded by manipulating the MOT laser detuning and hy-perfine pump power[16].The FORT had a trap depth of120µK with trapping frequencies of450Hz radial by35Hz axial with a typical gas cloud temperature of 15µK.Standard detunings during the last stage of load-ing the optical trap for the MOT cooling lasers were72 MHz and60MHz to the red of the cycling transition for 87Rb and85Rb,respectively.Turning the FORT light on and offwas performed using an acousto-optical modula-tor(AOM).After the atoms were loaded into the FORT, all other light(MOT and repump lasers)was shut offand the atoms were held for100ms in the FORT to allow for equilibration.Imaging was accomplished through stan-dard absorptive imaging techniques.With our param-eters3.5million87Rb atoms or4.5million85Rb atoms could be loaded individually.However,when simultane-ously loaded the number dropped to around2million for each isotope.This reduction is indicative of cross-species light assisted collisions.Once the atoms were prepared in the FORT,we illumi-nated them with a pulse(“flash”)of laser light to induce light assisted collision loss.To drive light assisted colli-sion losses,one of the MOT cooling lasers at its standard detuning was used to couple atom pairs from the ground state to a selected excited state potential(85Rbflash in-tensity was15mW/cm2and87Rbflash intensity was25 mW/cm2).Typicalflash time was4ms,but data ex-tending over a range offlash times from0.5ms to20ms were examined.The main trapping and repump MOT lasers made up theflashing lasers,thus creating an opti-cal molasses.While the complicated polarization struc-ture of an optical molasses is undesirable for these mea-surements,using a single beam of comparable intensity would produce too much recoil heating to make effective measurements.We found that there was an elevated ini-tial loss associated with thefirst few hundredµs of the flash,while the atoms were being hyperfine pumped.In order to avoid these complications,we used a0.5msflash to establish a baseline and then used longerflashes to measure the loss from that point.An additional complication fromflashing the atom cloud with an optical molasses came in the form of“me-chanical heating”of the cloud.The high density of the atoms in the upper hyperfine ground state in the trap can lead to a significant heating of the gas due to rescat-tering effects[25,26],depending on the detuning of the pulse.This could lead to density dependent losses due to subsequent evaporative cooling from the optical trap if the atoms are held there long enough,mimicking a light assisted collision loss.One way this potential sys-tematic uncertainty was mitigated through our choice ofFIG.2:Excited state potentials for homonuclear light as-sisted loss.The zero of the scale is arbitrary.Transitions are accessed from initial ground hyperfine states of:(a)and (b)85Rb F=3,(c)and(d)87Rb F=2.Both(a)and(b)are accessed with aflash light which is detuned by60MHz to the red of the85Rb cycling transition,where as(c)and(d) are accessed with aflash light which is detuned by72MHz to the red of the87Rb cycling transition.In contrast to Fig. 1the lettering scheme in this plot refers to specific hyperfine states rather than hyperfine manifolds.The horizontal red lines depict laser frequency used to access each transition.flash laser detuning.We also used a two image subtrac-tion technique to measure atom loss that involved hold-ing the atoms in the optical trap for only a short(∼5 ms)time compared to the elastic scattering time.In this technique,thefirst image was taken while the FORT was held on(in-trap).The second image was taken after the FORT was turned off(out-of-trap)after a5ms free ex-pansion time.The in-trap atom count,excluding the FORT region,was then subtracted from the out-of-trap image and this properly accounted for the atoms that had remained in the FORT after theflash,without having to wait until the atoms lost from the FORT had completely fallen away from the imaging region.In order to confirm that we were observing density de-pendent losses,we took data with a single isotope(85Rb) that examined the number remaining in the trap as a function offlash time(Fig.3).A one body lossprocess FIG.3:Log of85Rb atom number vsflash time.The dashed line is afit to the curve assuming two body loss while the straight line is afit to the last three points.There is a clear deviation of the number evolution from a straight linefit to the last data points.would appear as a straight line in Fig.3,and since our data do not follow a straight line we confirmed that we were measuring density dependent losses.The number remaining as a function offlash time combined with the measured density of the atoms in the FORT allow for the two-body loss rate(K2)to be extracted.Equation (1)and(2)define the differential equations for our mea-sured loss rates.d3xdn F85dt=−K F i2(85−85)d3x(n F85)2−K F F i2(85−87)d3x n F85n F 87(1)d3xdn F 87dt=−K F i2(87−87)d3x(n F 87)2−K F F i2(85−87)d3x n F85n F 87(2)Where K F i2(85−85),K F i2(87−87),and K F F i2(85−87)are the light assisted collisional loss rates for homonuclear85/85, 87/87,and the interspecies85/87respectively;i is a label forflash light frequency used.n85,n87are the85and87 densities.F,F’are the ground hyperfine states involved in the collision for85and87respectively.We examined all the transitions which could be reso-nantly excited under our experimental conditions,except mixed homonuclear ground state distributions.All the measured K2values are reported in Table1.In addition to the statistical uncertainties shown in the table,there is an additional overall uncertainty of40%in the abso-lute values of the rates due to uncertainy in our density calibration.While two of the measured rates are consis-tent with zero,the rest of the rates are the same order of magnitude but are different by factors of∼2.These4measured rates are much higher than those measured in MOT light assisted collisions for comparable laser inten-sities and that is expected given the shallow nature of the trap.The most straightforward collision rates to compare at the qualitative level are those associated with the poten-tials shown in Fig.1(a-b).Unlike the other cases,the long range excited state potentials are purely repulsive (1a)or attractive(1b).In previous experiments with photoassociation and light assisted collisions,repulsive potentials were used with“optical shielding”to reduce collision rates[27,28,29,30,31,32].However,those ex-periments relied on a resonant excitation with the shield-ing light where the pair could only gain a maximum ki-netic energy which was less than the trap depth,and thus not lost from the trap.For our parameters,a reso-nant excitation can impart∼10times the trap depth in kinetic energy;in previous experiments these conditions led to additional loss[33,34].Therefore,if the collision rate is controlled by resonant excitations the loss rate for purely repulsive and attractive potentials should not be markedly different for our parameters.Our observa-tions,however,show the loss rates for the purely repul-sive potentials are significantly lower than for the purely attractive potentials.This degree of suppression of the loss rate is not ex-pected in a semi-classical model of the collision that takes only the excitation rate to the excited state po-tentials into account.As an example,we performed a loss rate calculation using the Gallagher-Pritchard(GP) model[35](even though not all the requisite assumptions apply in our parameter range).In making this calcula-tion,we included only radiative escape losses and deter-mined the survival probability for excitation at a given internuclear radius by explicitly integrating the motion of atom pairs on a representative excited state potential to find the time the pair would require to accelerate to the trap escape velocity.These GP model calculations did not reproduce our observed loss rates.For the detunings used here the GP model gives a loss rate for the purely re-pulsive potentials which is an order of magnitude greater than that measured.Additionally,the model predicted the purely repulsive potentials would yield a comperable loss to the attractive potentials.A better description of the collision dynamics for these potentials can likely be obtained by using a dressed state picture and examining Landau-Zener(LZ)crossing prob-abilities.Briefly,as the atoms approach one another dur-ing a collision,they encounter an avoided crossing created at the value of internuclear separation R c(Condon ra-dius)where the light resonantly couples the ground and the excited state.At this avoided crossing,the atoms can either remain in the ground state or adiabatically trans-fer to the excited state,which could ultimately result in trap loss.This LZ approach has been shown to accurately reproduce more sophisticated theoretical treatments for both attractive[36]and repulsive[37]potentials. According the LZ theory[38,39],the probability for making a diabatic crossing isP=exp[− Ω22παv](3) WhereΩ=Γ(I/I sat)/2(Γis the natural linewidth),αis the slope of the potential curve at the Condon radius, and v is the velocity of the atom pair.Fig.4(a-b)shows an example of the LZ crossing for the repulsive(4(a)) and attractive(4(b))potential curves.By comparing the sequence of adiabatic and diabatic crossing that result in the loss in the attractive and repulsive potential cases, we can formulate an LZ prediction for the ratio of those loss rates and compare them to our measurement.To estimate the ratio between the attractive and repul-sive case we model the numerous atom potentials with just one or two representative potentials.In our calcu-lation ofΩwe average over all possible light polariza-tions and include a Clebsh-Gordon coefficient based on the asymptotic hyperfine state character of the excited state potential.For our parameters,the value of P at the mean velocity is0.70and0.94for the outermost and innermost avoided crossings infig.4(a)and0.59for the avoided crossing shown infig.4(b).By tallying all of the possible crossings,determining which crossing sequences produce loss,performing a thermal average over all of the collision energies in the cloud,and using equation(3)to estimate the diabatic crossing probability at each avoided crossing,the ratio of the loss probability in the attractive case to the loss probability in the repulsive case can be calculated.Wefind that this ratio of attractive to repulsive loss probability is1.6.This ratio was obtained ignoring spon-taneous emission at R c and hyperfine changing collisions near R=0.When these spontaneous emission losses hy-perfine changing collision events are estimated and in-cluded,the ratio does not change significantly,going to 1.3.The reason for this insensitivity is that approaching the attractive potential case avoided crossing from R=0 is very similar to approaching the outermost repulsive potential avoided crossing from R=∞,and so increas-ing the loss at these avoided crossings increases the loss probability for both the attractive and repulsive potential case.While this ratio of probabilities suggests that the at-tractive potential should produce a larger loss rate than the repulsive,a factor of1.6is inconsistent with our measured rates at the95%confidence level[40].There are several factors that could explain this disagreement. First,the LZ model calculates a probability of loss but in order to produce a loss rate an incomingflux needs to be specified as well.Based on the fact that R c is simi-lar for both the repulsive and attractive curve cases,from purely geometric considerations the incomingflux should5Loss table87F=285F=3 (·10−10cm3/sec)72MHz Red60MHz Redof the87cycling of the85cycling 87F=2 6.92(0.52):2(c)0.48(0.35):1(a)87F=1- 2.36(0.68):1(b)85F=3 2.22(0.57):1(c) 4.75(0.40):2(a)85F=20.61(0.99):1(d)-TABLE I:Measured K2rates.The isotope and initial hy-perfine ground state of each atom in the collision is specified. Also,theflash light used to induce the loss is specified as well. The labels for each measured loss rate refer to the specific excited state potentials shown in Figs.1-2.The numbers in parenthesis indicate the statistical uncertainties for each measurement.be similar.The collision rates ultimately should be cal-culated quantum mechanically,though,and that gives the opportunity for destructive and constructive inter-ferences to arise.For instance,during some collisions in the attractive potential case atom pairs will make multi-ple transits between R=0and R=R c as they are reflected at R=0and at the avoided crossing.The ultimate out-wardflux of these oscillating atom pairs depends on ac-quired phases that are not included in our simple model. Also,for our parameters the approximation of reducing the numerous potential curves to a single potential curve is not severe if only average LZ crossing probabilities are considered.However,this reduction will remove inter-ference effects arising from multiplecrossings[41,42,43]. We note,though,that our thermal average and magnetic sublevel distribution would likely wipe out some of these interference effects.Beyond these interference considerations,problems with this simple LZ picture can also arise because of the assumption of average polarization.In reality,the atom magnetic sublevel distributions and the light polarization are not uncoupled,and optical pumping will correlate the atom states and the light polarization.This can pro-duce different effective values ofΩfor the repulsive and attractive cases;though estimates of the impact due to this optical pumping shouldn’t change the ratio by more than20%.Additionally,central to the LZ assumption is that v is constant during the crossing and that the ac-tual potentials can be modeled by replacing them with the appropriate tangent lines at R c.Given that the po-tentials for our parameters near R c are not as sharp in an absolute sense as in other experiments[27,29,33,34], these assumptions may be more questionable in our work. Our main conclusion is that even for the potentials with the simplest structure,neither the GP model nor the LZ model reproduce the observed ratio of loss rates between the attractive and repulsive cases.Thus,the dynamics of the collision appear to depend sensitively on the details of the potentials,even for purely attractive and repuslive potentials.While it is relatively straightforward to make compar-FIG.4:Representative dressed state potentials used in the LZ calculation of relative loss probabilities.The zero of the energy scale is selected to correspond to the bare ground state energy at R=∞.The topfigure corresponds to the purely repulsive potential case(corresponding to Fig.1(a))and the bottomfigure corresponds to the purely attractive potential case(corresponding to Fig.1(b)).In the topfigure,a sample shielding and sample loss sequence of crossings in indicated. In the bottomfigure,a sample loss sequence is shown. isons between Fig.1(a-b)due to the simplicity of the ex-cited state potentials,the other accessible excited states have much more complicated structure.In particular, when mixing both attractive and repulsive potentials many avoided crossings are generated in the potentials themselves,as shown in Fig.5.Thus some potentials which are initially attractive during the collision can be-come repulsive at short range and vice versa,leading to complex dressed state potential curves.While detailed calculations in this system would be difficult,it is rea-sonable to expect that the presence of repulsive potential curves could mitigate the loss rate.The repulsive curves can turn colliding atom pairs away from short internu-clear radii and slow initially accelerated atoms pairs,re-ducing the loss rate from what it would otherwise be. Comparing the loss rates associated with the poten-tials in Fig.1(c-d)is suggestive of this.The loss rate for the potential associated with Fig.1(d)is less than that for the potential represented in Fig.1(c).One difference between the two potentials is that the one in Fig˙1(d) is shorter ranged,leading to an expectation of less loss6based on the number of atom pairs that collide with suf-ficiently low impact parameter.In addition,the avoided crossing structure in Fig.1(d)is much sharper,leading to steeper repulsive potential curves,from which a miti-gation of the loss rate would be expected.Fig.2(a-d)show homonuclear excited state potentials for85Rb alone(2(a-b))and87Rb alone(2(c-d)).Sim-ilar to the heteronuclear potentials shown in Fig.1(c-d),these potentials too have a mixture of attractive and repulsive potentials.However,the homonuclear poten-tials are longer ranged than the heteronuclear ones,as expected from the1/R3asymptotic nature of a homonu-clear potential as compared to a1/R6asymptotic nature of a heteronuclear potential.This longer range would suggest,all other things being equal,a larger light as-sisted collision cross section.Indeed the measured loss rates(see Table1)indicate that the loss rates are larger for the homonuclear case.Though it is interesting to note that the loss rate for homonuclear85Rb collisions is lower than that for homonuclear87Rb collisions,de-spite the Fig.2(a)potential and the Fig.2(c)potential being qualitatively similar and expected to be the po-tential curves most directly important for the loss.We speculate that this difference is produced from a combi-nation of the difference in the relevant Clebsch-Gordan coefficients for the transitions and the presence of the re-pulsive potentials in the85Rb which extend further out closer to resonance allowing for a higher“self-shielding”probability.Once again,the lower loss rate is associated with the potential with the more repulsive character. An interesting effect observed in the homonuclear data was that the loss rate was saturated at ourintensities[44, 45,46,47,48].To measure saturation,we cut the laser intensity by1/2during theflash.The ratio of the1/2in-tensity loss to the full intensity loss is then computed[49]. Without saturation effects,the loss should scale linearly with the light intensity.With85and87homonuclear loss,the ratio of the measured K2at half intensity to the rate measured at full intensity is0.98(12)and1.08(12) respectively.The fact that no change was observed in-dicates that the losses are severely saturated at our trap intensities.Given the reported results for photoassociation satura-tion,wefirst examined whether or not the unitarity limit could be responsible for the observed saturation[46,47, 48].A classical estimation at the most probable collision energy in the cloud indicates that contributions up to h-wave are significant.Including up to h-wave produces a unitarity limit of∼36·10−10cm3/sec;much higher than the loss rates measured.This indicates that unitarity is not the cause of the observed saturation.Furthermore, in the unitarity limited regime the scattering rate should not distinguish between85Rb and87Rb,yet the homonu-clear loss rate saturates at different collision rates which suggests the details of the potential must play a role in de-termining the loss rate.Additionally,the heteronuclear FIG.5:Excited state potential avoided crossings.This plot shows two regions of the excited state heteronuclear poten-tials in more detail to indicate the complicated avoided cross-ing structure of these potentials.The top plot is the just the potential shown indicated infig.1(c)with its x-and y-axes rescaled.The bottom plot is the same for the potential indicated infig.1(d).loss rate doesn’t appear to saturate and a decrease in the flash intensity by half seems consistent with a decrease in the loss rate by half,as the ratio of the heteronuclear loss of half intensity to full intensity was measured to be 0.60(20).Rather than unitarity,our results seem consistent with thefinite pair formation rate of the atoms in the cloud which is referred to as“ground state depletion”in the literature[50].A classical hard sphere estimate for our experimental conditions shows that for a required close approach internuclear distance of R=72nm,the maxi-mum pair formation rate is7·10−10cm3/sec;consistent with the observed loss rate.While this is just an esti-mate,it along with the observed saturation indicates that ground state depletion likely plays a role in these colli-sions.Since the observed saturation rates are different, the homonuclear potentials must induce some dynamics which alter the collision rates,however.This would be consistent with a model where not every atom pair that collides at the critical radius suggested by ground state depletion is lost.。
高中英语作文介绍一种节约能源的方法
高中英语作文介绍一种节约能源的方法Title: A Method of Energy SavingWith the increasing demand for energy and the corresponding environmental concerns, it is crucial for us to find ways to save energy.Among the various methods, using energy-saving appliances is an effective way to reduce energy consumption.The first advantage of energy-saving appliances is that they consume less energy compared to traditional appliances.For example, energy-saving light bulbs consume about 75% less energy than traditional light bulbs.This reduction in energy consumption not only helps to save electricity bills but also contributes to the overall energy saving efforts.Secondly, energy-saving appliances are designed to be more durable and long-lasting.This means that they do not need to be replaced as frequently as traditional appliances, which in turn reduces the amount of energy wasted during the production and disposal of these appliances.Lastly, using energy-saving appliances is also beneficial for the environment.By reducing energy consumption, we can reduce the amount of greenhouse gas emissions that contribute to climate change.Additionally, less energy consumption means less demand for fossil fuels, which can help to preserve natural resources and reduce environmental pollution.In conclusion, using energy-saving appliances is an effective andpractical way to save energy.Not only can it help to reduce energy consumption and lower electricity bills, but it also has environmental benefits.Therefore, it is important for us to choose energy-saving appliances and promote their use to contribute to a sustainable future.。
学术综合英语unit2
Energy in TransitionThe era of cheap and convenient sources of energy is coming to an end. A transition to more expensive but less polluting sources must now be managed.John P. Holdren能源转型能源资源价格低廉、使用便捷的时代已经过去，目前应向尽管价格较高、但污染较小的资源转变。
约翰·P·霍德雷恩Understanding this transition requires a look at the two-sided connection between energy and human well-being. Energy contributes positively to well-being by providing such consumer services as heating and lighting as well as serving as a necessary input to economic production. But the costs of energy －including not only the money and other resources devoted to obtaining and exploiting it but also environmental and sociopolitical impacts －detract from well-being.了解这一转变，需首先考察一下能源和人类幸福的双重关系。
从积极的意义上说，能源为人类幸福作出了贡献，它为经济生产活动提供必要投入的同时，也提供了诸如取暖、照明等消费服务。
然而，人类为利用能源所付出的代价却削弱了能源为其带来的利益，这种代价不但包括为获取和利用能源所投入的资金和其他资源，而且包含了能源开发和利用所产生的环境影响和社会政治影响。
Improving a radiative plus collisional energy loss model for application to RHIC and LHC
a r X i v :n u c l -t h /0701088v 2 20 J u n 2007Improving a radiative plus collisional energy loss model for application to RHIC and LHCSimon Wicks,Miklos GyulassyColumbia University,Dept of Physics,538West 120th Street,New York,NY 10027E-mail:simonw@Abstract.With the QGP opacity computed perturbatively and with the global entropy constraints imposed by the observed dN ch /dy ≈1000,radiative energy loss alone cannot account for the observed suppression of single non-photonic electrons.Collisional energy loss is comparable in magnitude to radiative loss for both light and heavy jets.Two aspects that significantly affect the collisional energy loss are examined:the role of fluctuations,and the effect of introducing a running QCD coupling as opposed to the fixed αs =0.3used previously.1.IntroductionNon-photonic single electron data [1,2],which present an indirect probe of heavy quark energy loss,have significantly challenged the underlying assumptions of jet tomography theory.A much larger suppression of electrons than predicted [3]was observed in the p T ∼4−8GeV region.“These data falsify the assumption that heavy quark quenching is dominated by [pQCD based]radiative energy loss when the bulk [weakly coupled]QCD matter parton density is constrained by the observed dN/dy ≈1000rapidity density of produced hadrons.”[4]WHDG [4]revisited the assumption that pQCD collisional energy loss is negligible compared to radiative energy loss [5,6].As argued there,and references therein,“the elastic component of the energy loss cannot be neglected when considering pQCD jet quenching.”As shown in WHDG and elsewhere [7],the computationally expensive integrations over the geometry of the QGP cannot be reduced to a simple ‘average length’prescription.Indeed,this computation time is essential to produce radiative +collisional energy loss calculations consistent with the pion data.There are large theoretical uncertainties in the WHDG results [8].Very significant to the electron prediction is the uncertainty in the charm and bottom cross-sections.There are also theoretical uncertainties in the energy loss mechanisms.Here,two aspects of the collisional energy loss will be examined with the aim of improving the energy loss model.2.Collisional energy loss fluctuationsSimilar to radiative energy loss,the fluctuations of collisional energy loss around the mean affect the quenching of the quark spectra.Collisional fluctuations are often modelled in a Fokker-Planck formalism,characterized by two numbers or functions:drag and diffusion.WHDG implemented an approximation to this scheme applicable for small energy loss by giving the collisional loss a gaussian width around the mean,with σ2=2T ǫ ,where ǫ is the mean energy loss given by a leading log calculation.The drag-diffusion method is essentially a continuum approximation to a discrete process.A high energy jet traversing the QGP will undergo only a small number of collisions.In the Gyulassy-Wang model,the expected mean free path of a quark is ∼2fm,so there is a very significant surface region in which the fluctuations will differ greatly from those given by the continuum approximation.It is therefore necessary to look at the fluctuations per collision and in the number of collisions.A simple model to investigate this is to model the medium as initially static objects which will then recoil upon collision,model the interaction between jet and medium using the full HTL medium modified propagator.This gives the probability of longitudinal momentum loss:dN 4πC R C 2E +mC L |∆L |2+C T |∆T |2C L =2+1m),C T =−ω(n +1)!dx 1...dx n ρ(x 1,E )...ρ(x n ,E )ρ(ǫ−x 1−...−x n ,E )(2)The mass of the medium particle is tuned to give an average energy loss similar to that of the BT and TG leading log calculations (m ∼0.2GeV -although here we are not interested in the average energy loss per se).In Fig.1,the probabiliy of fractional energy loss in one 2style model,with screening at Figure 1.The distribution of fractional energy loss ǫ(left)and Mandalstam variable t (right -scaled by t 2)in a collision using this model.Figure 2illustrates the distributions in energy loss for a finite number of collisions for bottom and light quark jets.The results for charm quarks are qualitatively similar to those for light quarks.For a large number of collisions (eg average number of collisions n =10,L ∼20fm),the distributions are roughly symmetric and somewhat similar to the simple WHDG gaussian.This is expected from the central limit theorem.The R AA values extracted from these distributions are similar,with n =10and the gaussian approximation only differing by ∼0.01.Surprisingly,a similar result for the R AA values is found for n =2collisions for bottom quarks.The large change arrives for light quarks.For both n =2,5collisions,the gaussian approximation gives a very different distribution for the fluctuations and a very different R AA value.TheFigure2.The distribution in fractional energy loss for a20GeV jet,for a bottomquark jet and a light quark jet,for different numbers of collisions.Shown are lines forthe gaussian approximation,exactly n collisions(labelled eg‘n=10’)and on averagen collisions(labelled eg‘ n =10’).Inset is the R AA for these distributions and theR AA evaluated for a delta function distribution at the mean loss(+point). gaussian approximation overpredicts the R AA suppression by0.1,which is around a 30%effect for n =2collisions.This cannot be neglected.A full treatment of thefinite number of collisions will reduce the quenching due to elastic energy loss compared to the treatment in WHDG.This conclusion is also applicable to other uses of Fokker-Planck /Langevin formalisms that use a continuum description of the collisional process.The R AA predictions for bottom quarks are likely only marginally affected,those for light quarks most affected.3.Running QCD couplingIn[8],the change of thefixed QCD couplingαs from0.3to0.4was seen to significantly change the R AA precitions from the WHDG model.There has been much recent work on the effect of a running coupling on the collisional energy loss[9,10](ieαs=αs(Q2)). Here,we revisit the collisional energy loss in a similar manner to[9],looking at a simple Bjorken-style estimate[11].Bjorken’s estimate for the collisional energy loss is:dE q,g3 ±12π 1+1µ2 (3)In[9],the running coupling version for very high jet energies is given as:dE q,g3 ±12π 1+1b0T2(4)although this neglects thefinite energy kinematic bound on the jet.Adding in this bound to this calculation givesdE q,g3 ±12π 1+1µ2 (5)which is similar in structure to the originalfixed coupling estimate.A numerical comparison of equations3,4,5is shown in Fig. 3.For reasonable temperatures, T∼0.2−0.3GeV,all results are of a similar order of magnitude.For reasonable energies,no qualitatively new behavior is seen(although,as found in[9],the E→∞behavior is new,but this affects much higher energies than those of interest at RHIC or even LHC).When the kinematic bounds are taken into account,the result for the average energy loss including running coupling is often larger than thefixedαs=0.3result used in [4].However,the numerical result is very sensitive to the input parameters used,for µfrom Figure 3.Left:the average energy loss for a light quark jet evaluated for (1)Peshier’s running αs =αs (Q 2)but infinite energy jet approximation (2)Finite energy running αs ,(3)fixed αs independent of Q 2but evaluated at Q 2=(2πT )2and (4)fixed αs =0.3.The functional form of αs =αs (Q 2)is taken for vacuum at 1-loop as in [9].Middle and right:the ratio between the different versions for two different evaluations of the Debye screening scale µ,(1)‘self-consistent’µ[9]and (2)µevaluated at a fixed temperature.‘Bjorken (2)’is with all parameters evaluated with αs =0.3.4.ConclusionIt has been argued previously “that radiative and elastic average energy losses for heavy quarks were in fact comparable over a very wide kinematic range”[4],and even “E ≥10GeV light and charm quark jets have elastic energy losses smaller but of the same order of magnitude as the inelastic losses”[4].Hence,collisional energy loss cannot be neglected when considering jet quenching of high energy jets in the QGP at either RHIC or LHC.A simple model combining collisional and radiative energy losses significantly reduces the discrepancy between the predictions and data.Two possible improvements to the WHDG model have been examined here.The inclusion of a finite number of collisions is seen to reduce the effect of the collisional energy loss on the quenching of gluons,light and charm quarks,but not to significantly affect the bottom quark R AA .Opposite to this effect,including a running QCD coupling increases the energy loss by up to a factor of 1.5.The combination of these two affects,along with other large uncertainties in the prediction for electron R AA such as the ratio of charm to bottom total cross-sections,hints at the possibility that both the pion and electron R AA s may both be within range of purely perturbative calculations.References[1]S.S.Adler et al.[PHENIX Collaboration],Phys.Rev.Lett.96,032301(2006)[2]X.Dong,AIP Conf.Proc.828,24(2006)[Nucl.Phys.A 774,343(2006)][arXiv:nucl-ex/0509038].[3]M.Djordjevic,M.Gyulassy,R.Vogt and S.Wicks,Phys.Lett.B 632,81(2006)[nucl-th/0507019].[4]S.Wicks,W.Horowitz,M.Djordjevic and M.Gyulassy,Nucl.Phys.A 784,426(2007).[5]M.Gyulassy,P.Levai and I.Vitev,Nucl.Phys.B 594,371(2001)[arXiv:nucl-th/0006010].[6]M.Djordjevic and M.Gyulassy,Nucl.Phys.A 733,265(2004)[arXiv:nucl-th/0310076].[7]T.Renk and K.J.Eskola,[hep-ph/0610059].[8]S.Wicks,W.Horowitz,M.Djordjevic and M.Gyulassy,Nucl.Phys.A 783,493(2007).[9]A.Peshier,Phys.Rev.Lett.97,212301(2006)[hep-ph/0605294].[10]J.Braun and H.J.Pirner,arXiv:hep-ph/0610331.[11]J.D.Bjorken,FERMILAB-PUB-82-059-THY。
Energy Dependence of Jet Quenching and Life-time of the Dense Matter in High-energy Heavy-i
a r X i v :n u c l -t h /0405029v 2 12 M a y 2004Energy Dependence of Jet Quenching and Life-time of the Dense Matter inHigh-energy Heavy-ion CollisionsXin-Nian WangNuclear Science Division,MS70R0319,Lawrence Berkeley National Laboratory,Berkeley,CA 94720(April 20,2003)LBNL-57533Suppression of high p T hadron spectra in high-energy heavy-ion collisions at different energies is studied within a pQCD parton model incorporating medium induced parton energy loss.The p T dependence of the nuclear modification factor R AA (p T )is found to depend on both the energy dependence of the parton energy loss and the power-law behavior of the initial jet spectra.The high p T hadron suppression at√s =200GeV is about 30timeshigher than that in a cold nucleus [14,15].The observed strong suppression of high p T hadrons at RHIC is in sharp contrast to the results of P b +P b collisions at the SPS energy.Data from the WA98[17]experiment show no or little suppression of high p T (up to about 4GeV/c )pions [18,19].Even if one takes into ac-count the possible uncertainty in the reference p +p data [20],the suppression allowed by the data is still signifi-cantly less than in Au +Au collisions at RHIC,while the total charged multiplicities or the inferred initial parton densities only differ by a factor of 2.This implies that additional physics is at play in the energy dependence of the suppression of single hadron spectra in high-energy heavy-ion collisions.It could be the energy dependence of the Cronin effect due to initial state multiple scat-tering,the thermalization time and finite lifetime of the dense matter that limits the parton energy loss.The en-ergy dependence of the hadron suppression at large p T was predicted in Ref.[9]and also was recently studied in Ref.[16],with parton energy loss proportional to theobserved hadron multiplicity.In this brief report,we explore the sensitivity of the final high p T hadron suppression to the lifetime of the dense matter as well as the dependence on the collid-ing energy.We will study the high p T hadron suppres-sion at√s =62.4GeV and study the constraint on the lifetime by the measurement of hadron spectrum suppression.II.ENERGY DEPENDENCE OF HIGH P THADRON SUPPRESSIONWe will use a LO pQCD model [18]to calculate the in-clusive high-p T hadron cross section in A +A collisions,dσh AAπz cdσmodification factor given by the new HIJING parame-terization[22].The initial transverse momentum distri-bution g A(k T,Q2,b)is assumed to have a Gaussian form with a width that includes both an intrinsic part in a nucleon and nuclear broadening.Detailed description of this model and systematic comparisons with experimen-tal data can be found in Ref.[18].The effect of parton energy loss is implemented through an effective modified fragmentation function[23],D h/c(z c,Q2,∆E c)=(1−e− ∆Lz cD0h/c(z′c,Q2)+ ∆Lz cD0h/g(z′g,Q2) +e− ∆LdL 1dτmaxτ0dττ−τ0dL 1d=ǫ0(E/µ−1.6)1.2/(7.5+E/µ),(5)according to the numerical results in Ref.[29]in whichthermal gluon absorption is also taken into account in thecalculation of parton energy loss.Fit to the most cen-tral Au+Au collisions at√N binary dσh pp(6)for charged hadron(solid lines)and neutral pions(dashedlines)in central Au+Au(P b+P b at the SPS energy)collisions at different energies,from SPS√s=5.5TeV.Here,N binary = d2bd2rt A(r)t A(| b− r|)(7)is the number of geometrical binary collisions at a givenrange of impact parameters.At the SPS energy,the ob-served nuclear modification factor in central P b+P b col-lisions is consistently about1due to strong Cronin effectvia initial multiple parton scattering,leaving not muchroom for large parton energy loss[19].We shall returnto this point later.In central Au+Au collisions at RHIC,however,strongsuppression of high p T hadrons is observed.This can beattributed to large parton energy loss that overcomes themodest Cronin enhancement as observed in d+Au col-lisions[5–8]and gives rise to the large hadron suppres-sion.The energy-dependence of the parton energy lossin Eq.(5)describes well theflat p T dependence of thenuclear modification factor R AA(p T)for neutral pions atlarge p T in Au+Au collisions at√energy dependence of the parton energy loss in Eq.(5)and the power-law behavior of the initial jet -ing the same energy dependence of the parton energy loss but with a reduced amplitude due to smaller initial gluon density at√s =200GeVat high p T >10GeV/c .This is simply a consequence of the energy dependence of jet spectrum shape.The initial jet spectra at√s =5.5TeV is smaller than at 200GeV in the intermediate p T region,due to larger initial gluon density.However,the modification factor R AA (p T )increases with p T due to the flatter power-law spectra of jet production atLHC.101110FIG.1.Nuclear for charged hadrons (solid)and neutral pions (dashed)in 0-5%central Au +Au collisions at√s =17.2GeV.For other energies τf is assumed tobe larger than the system size.The STAR [31]and PHENIX [32]data are for central Au +Au collisions at√s =200GeV andapproaches its p +p value at p T >5GeV/c .This gives rise to the splitting of the suppression factor for charged hadrons and π0in the calculation.Because of the steeper power-law spectra of jet production at 62.4GeV,the ef-fect of the non-perturbative parton recombination per-sists to higher p T than in Au +Au collisions at 200GeV.In this region of p T ,the non-perturbative recombination effects dominate the nuclear modification of the charged hadron spectra.As a consequence,the p T dependence of the modification factors R AA (p T )at√s =200GeV,theextracted energy loss points to an initial gluon density of about 30/fm 3at an initial time τ0=0.2fm.Given the measured transverse energy per charged hadron of 0.8GeV [37],this gives a lower bound on the initial energy density of about 25GeV/fm 3.In 1-d expansion with the equation of state of an ideal fluid,the energy den-sity decreases with time,ǫ(τ)=ǫi (τi /τ)4/3.Assuming the 1-d hydrodynamics expansion starts at τi =1fm/c (free-streaming before that),the lifetime of the plasma or the duration for the parton energy loss should be about τf ∼5fm,before the phase transition with a critical energy density ǫc ∼1GeV/fm 3.The early stage of the mixed phase or crossover could also contribute to the jet quenching and thus extend the effective time duration for parton energy loss.When this time is larger than the 3average path length,the total parton energy loss is then limited only by the system size.In the previous analysis [14,29],such an assumption for central Au+Au collisions is justified given the high initial energy density.In central P b+P b collisions at the SPS energy√s=200GeV [30].The average transverse energy per charged particle is about the same at SPS and RHIC energy[37].One can then assume the initial energy density at SPS to be half of that in central Au+Au collisions at√s=62.4GeV.The lifetime of the dense medium isτf=10,4,3,2,1,0fm/c(from bottom to top).For central collisions results with only large values of τf are presented.The dot-dashed line in thefirst panel is for π0in0-10%central collisions withτf=10fm/cTo study the sensitivity of the high p T hadron suppres-sion to the lifetime of the plasma at√s=62.4GeV.The hadron suppression in the large p T region in the most central col-lisions is very sensitive to the lifetime of the plasma in this calculation.In peripheral collisions,the small size of the dense medium limits the parton energy loss.As a result,the hadron suppression is only sensitive to values ofτf that are smaller than the average medium size.In reality,the values ofτf should decrease from central to peripheral collisions.The recent experimental results from PHOBOS[40] on nuclear modification factors for charged hadrons in Au+Au collisions at62.4GeV only extend to p T∼4GeV/c.In this region,the suppression of charged hadron is indeed much smaller than at200GeV.How-ever,charged hadrons in this region are also dominated by non-perturbative recombination effects,though our results are still sensitive to the lifetime.Experimental measurements ofπ0and high p T charged hadrons,both are less influenced by the parton recombination effect, should provide more stringent constraints on the lifetime of the dense matter.IV.SUMMARY AND DISCUSSIONWithin a parton model incorporating medium induced parton energy loss,we have studied in this brief report the suppression of inclusive hadron spectra at high p T in heavy-ion collisions at different energies.We found that the p T dependence of the nuclear modification fac-tor R AA(p T)is determined by the energy dependence of the parton energy loss and the power-law behavior of the initial jet spectra.With the onset of parton energy loss and the change of the power-law jet spectra,the p T de-pendence of the modification factor changes from mono-tonic decrease at√s=5.5TeV.Theflat p T dependence observed at√central Au+Au collisions is almost identical to that at 200GeV.This is partly due to the trigger bias that se-lects dihadron production close to the surface and results in completely suppression the back-side jets that traverse the whole length of the dense matter.The suppression due to k T broadening of initial multiple parton scatteringis also independent of the colliding energy.The author thanks P.Jacobs for helpful discussion about this manuscript.This work was supported by the Director,Office of Energy Research,Office of High En-ergy and Nuclear Physics,Divisions of Nuclear Physics, of the U.S.Department of Energy under Contract No. DE-AC03-76SF00098and DE-FG03-93ER40792.V.APPENDIXIn this appendix,we give the basic analytic formula for calculating the path integral in the parton energy loss in Eqs.(3)and(4),assuming a hard-sphere nuclear distri-bution.Given two overlapping nuclei as illustrated in Fig.3, we want to calculate a path integral over the path∆L. Let r1and r2be the radial coordinates of the jet produc-tion point as measured from the center of the two nuclei. For given b and r1r2=R2A−r21sin2φr−r1cosφr;(10)τ2=R2min−r21sin2φb−r1cosφb,(14) where R min=Min(R A,R B).We also define R max= Max(R A,R B).Assuming that the soft gluon density is proportional to the number of participant nucleons,it is then given by ρg(τ, b, r)=τ0ρ02c AB R min R3ABt B(| b− r|)θ(R A−r),(15)where c AB=1−(1/2)(1−R2min/R2max)3/2andρ0is de-fined as the averaged gluon density in central collisions (b=0)at an initial timeτ0:ρ0=12πA1−r2/R2A,(17) the gluon density is thenρg(τ, b, r)=3τ0ρ0R2B−| b− r|2+θ(R B−| b− r|)dL 1d∆L+τ0τ0dττ−τ0dL 1d3τ×R2B−| b−( r+ nτ)|2.(19) The average number of scatterings is∆L/λ = τ0+∆Lτ0dτσρg(τ,b, r+ nτ)=3R min τ0+∆Lτ0dτR2A−( r+ nτ)2+The above integrals can be completed analytically. The following are some basic integrals:dτ 22arcsinτ+ r· nR2−r2+( r· n)2,(21)dτR2−( r+ nτ)2=R2−r2 log R2−r2−( r· n)τ+ R2−( r+ nτ)2 −logτ (22)。
Unit 13 课文原文Energy and Environment
Unit 13 Energy and EnvironmentPassage A Nuclear Energy Regulation Risk and the EnvironmentAbdullah Al Faruque1The linkage between energy and the environment is well established and undeniable as the use of any energy source has some effect on the environment albeit the degree of effect may vary depending on the particular form of energy used. The symbiotic relationship between energy and the environment can be further explained by the fact that use of non-renewable sources such as fossil fuels can emit carbon dioxide, which contributes to global warming. The international community is increasingly pursuing energy security and sustainable development through deployment of cleaner, more efficient and low-carbon energy technologies. Thus, in the energy sector, reduction of greenhouse gas emissions remains a main factor in choices about energy options for electricity generation. Although reduction of greenhouse gas emission is not the main driving force behind the current use of nuclear energy by the States, its potential role in promoting sustainable energy source will be of central importance in the coming decades.2The environmental aspects of nuclear power plants and the facilities of the associated fuel cycle are not very different from any other large-scale industrial activity. However, the radioactive materials that are part of the various fuel cycle operations, particularly those radioactive materials generated during the operation of nuclear reactors, have to be strictly controlled.3The growing global demand for energy, the issue of combating climate change and the gradual decline of dependence on fossil fuels have warranted a renewed emphasis on nuclear power. Nuclear energy is currently contributing about 17 per cent of the total global electricity production. Nuclear material and technology are also useful for medicine and agriculture. The justification for a nuclear revival has been based largely upon two policy priorities: climate change mitigation and security of energy supply.4Nuclear energy is often considered a clean nonrenewable energy source in terms ofemissions. From an emission standpoint nuclear energy is more environmentally friendly than coal, oil or gas. The importance of nuclear energy is increasing since it is capable of meeting a significant portion of the energy needs of a country. Thus, nuclear power should be considered as one of the significant options for meeting future world energy needs at low cost and in an environmentally acceptable manner. Nuclear energy has assumed growing significance as emission-free energy in an era of serious concern about global warming.5In order to improve public perception of the nuclear industry, the issue of safety and waste management needs to be further developed and addressed and the industry must continue to pursue a policy of non-proliferation of nuclear weapons.6Although nuclear technology is currently applied in diverse areas of human activity, such as medicine, nuclear research, agriculture and food preservation, the main risk stems from the generation of nuclear energy from the nuclear power plant. The nuclear power plant is the main part in the nuclear fuel cycle chain, and it is the place where the fission process occurs. Other parts of the nuclear fuel cycle include the transportation of nuclear materials and the management and transportation of spent fuel and nuclear waste.7Thus, the sources include all types of nuclear facilities, such as power reactors, research reactors, nuclear fuel cycle facilities, as well as medical, research and industrial sources, and defense-related sources where appropriate. After the Fukushima nuclear power plant accident 1, public concern about nuclear energy has increased significantly. There are widely varying perceptions of the risks and benefits of nuclear energy. The catastrophic nature of the risk of exposure from a nuclear power plant that can potentially bring great destruction and untold human suffering to humanity and the environment makes this risk unacceptable to humanity. The opposition to nuclear power plant has been expressed in the following ways: 8First, the long-term disposal of radioactive wastes remains a major challenge for the international community. A nuclear power plant creates spent nuclear fuel at the reactor site. Spent nuclear fuel is considered high-level waste that has many potential negative effects onthe environment. The resulting waste from use of nuclear energy can last thousands of years and can pose some danger to present and future generations. No state has found a solution to the problem of long-term disposal of nuclear waste.9Second, a major concern over nuclear energy is the long-term effects of radiation on the people living near or working in a nuclear power station. Although nuclear power plants emit low levels of radiation into the environment, long-term exposure to low-level radiation can be a health risk. While sources of ionizing radiation are essential to modern health care, they can be detrimental to living organisms if the production and the use of radiation sources and radioactive material are not covered by measures to protect individuals exposed to radiation. Ionizing radiation and radioactive substances have a permanent effect on the environment and the risks associated with radiation exposure can only be restricted, not eliminated entirely. Radiation protection from nuclear energy has become an important concern from the perspective of both human and environmental health.10Third, every operating nuclear power plant poses some risk of a severe or large-scale accident. But the risk of such accident is extremely low or insignificant. The nuclear industry estimates the chances of a severe reactor accident to be about one for every 10,000 reactor years of operation.11Fourth, nuclear power plants may not emit carbon dioxide during operation, but high amounts of carbon dioxide are emitted in activities related to building and running the plants. The process of mining the uranium which is used in nuclear power plants also releases high amounts of carbon dioxide into the environment. The mining needed to extract uranium may itself have some negative environmental impacts. Some carbon dioxide emissions occur in various stages of the nuclear fuel chain—mining, milling, transport, fuel fabrication, enrichment , reactor construction, decommissioning and waste management. Uranium mining and milling of uranium mill tailings have radioactivity and this remains after uranium is extracted by milling.12Another type of radioactive waste consists of tailings generated during the milling of certain ores to extract uranium or thorium. These wastes have relatively low concentrations of radioactive materials but they remain for long period of time. Thus, uranium mill tailings can adversely affect public health. Nuclear fuel is a kind of enriched uranium but plutonium is a by-product of nuclear power generation. Apart from uranium, which is the primary source of supply for nuclear energy production, plutonium from spent fuel and re-enriched tails from processing residues , stockpiles and ex-military weapons is a secondary source of supply.13Fifth, nuclear power has higher overall lifetime costs compared to natural gas and coal. The nuclear reactor is more expensive to build than conventional fossil fuel units. Thus, nuclear energy may be the most expensive way to produce electricity.14Sixth, the illegal trade in nuclear material and the proliferation of nuclear weapons is another global concern. Many countries are aspiring to nuclear energy and any increase in the number of states with nuclear energy capacity increases the likelihood of nuclear proliferation through weaponization of civilian nuclear energy materials. The current international legal framework is not fully adequate to eliminate the risk of such proliferation and to meet the security challenges of the expanded nuclear energy programme.15Seventh, transportation of radioactive material raises another public concern over the environmental impacts of such transport. Transport of nuclear fuel to and from nuclear power plants requires adequate packaging and regulatory measures to protect humans and the environment from the hazards of exposure to radiation. The volume of transportation of radioactive material is increasing rapidly and will continue to increase with the growth of the nuclear power industry.16Eighth, potential terrorist and caber-attacks and sabotage on nuclear power plants pose additional risks. There is a fear that nuclear weapons or enriched uranium or plutonium may reach terrorist groups who can make small and unsophisticated nuclear bombs. The possibility of diversion of nuclear material through terrorist acts cannot be ruled out. Furthermore, risksposed by human error and natural disasters can also be significant.17Finally, there are unknown and unpredictable safety and environmental risks associated with nuclear energy production that may have long-term consequences. Use of nuclear energy also raises public health concerns with regard to uranium mining and reactor safety, as well as transport and disposal of nuclear waste. Some epidemiologists point out the statistically significant increase of cancer among workers in the nuclear fuel cycle and people living close to nuclear waste reprocessing plants. Some public health scholars suggest that nuclear power plants expose people to "low-level ionizing radiation, with increased health risks attendant to this exposure". Harvard and MIT scholars have stressed that modern reactor designs can achieve a very low risk of serious accidents but have admitted that, although technological progress has made nuclear reactors safer, they are not totally risk free, and the risk of a reactor leak or other kind of accident can never be dismissed completely.18Thus, nuclear technology is seen as " inherently hazardous" given its potential for large-scale damage to human health and the environment. Although nuclear risk per se has a low probability that is difficult to estimate, its foreseen damages are of an extreme magnitude in the event that it occurs. In other words, whereas the risk of a nuclear catastrophe is low, its impact on public health remains unknown. Risks posed by nuclear energy production are very difficult or even impossible to quantify.19Although since the Chernobyl accident 2 the nuclear power industry has strengthened its safety practices and standards, some risks are inherent in nuclear energy. The scope of nuclear risk is now broader than merely the risk of nuclear accident. (1,620 words)。
如何阻止火山爆发英语作文
Volcanoes have long captured the imagination and fear of humanity, their eruptions a testament to the immense power of nature. As a high school student fascinated by geology and environmental science, Ive often pondered the question: How can we prevent volcanic eruptions? Its a complex issue that intertwines science, technology, and the delicate balance of our planets geological processes.Firstly, its important to understand that volcanoes are a natural part of Earths landscape, and their eruptions are part of the Earths natural cycle of creation and destruction. They release gases and lava, contributing to the formation of new land and the recycling of the Earths crust. However, the destructive potential of a volcanic eruption is immense, capable of causing widespread devastation, loss of life, and longterm environmental impacts.Preventing a volcanic eruption is not about halting a natural processits about managing the risks and mitigating the impacts. Heres how we can approach this challenge:1. Monitoring and Early Warning Systems: The key to managing volcanic activity lies in constant monitoring. Seismographs, gas detectors, and satellite imagery are used to detect signs of an impending eruption. By setting up an extensive network of sensors around active volcanoes, scientists can gather data on seismic activity, ground deformation, and gas emissions. This information is crucial for predicting eruptions and giving communities enough time to evacuate.2. Geothermal Energy Utilization: A fascinating approach to reducing thepressure in a volcanos magma chamber is through geothermal energy extraction. By tapping into the heat beneath the Earths surface, we can potentially relieve some of the pressure that leads to eruptions. This method, however, is still in the experimental stages and requires further research to understand its full implications.3. Lava Diversion: In some cases, it might be possible to divert the flow of lava away from populated areas. This could involve creating channels or barriers that guide the lava flow in a safer direction. However, this is a highly risky and complex operation that requires precise engineering and a deep understanding of the volcanos behavior.4. Public Education and Preparedness: One of the most effective ways to prevent the loss of life during a volcanic eruption is through education and preparedness. Communities living near volcanoes should be wellinformed about the risks, the signs of an impending eruption, and the evacuation procedures. Regular drills and community engagement can save lives.5. International Collaboration: Volcanoes do not respect borders. Effective management of volcanic risks requires international cooperation. Sharing data, technology, and expertise can help countries better prepare for and respond to volcanic eruptions.6. Research and Innovation: The more we learn about volcanoes, the better we can predict and manage their eruptions. Investing in research and innovation is crucial. This includes developing new technologies for monitoring volcanic activity and exploring novel methods for reducing thepressure in magma chambers.7. Policy and Legislation: Governments play a vital role in volcanic risk management. They can enact policies that restrict development in highrisk areas, ensure that buildings are constructed to withstand volcanic events, and provide funding for research and monitoring programs.8. Economic Incentives: Encouraging sustainable development around volcanoes can reduce the impact of eruptions. This might involve offering economic incentives for building in safer areas or investing in infrastructure that can withstand volcanic events.In conclusion, while we cannot stop volcanic eruptions, we can take steps to minimize their impact. By combining advanced technology, scientific research, community engagement, and international cooperation, we can better prepare for and respond to these natural phenomena. The goal is not to conquer nature but to live in harmony with it, understanding its power and respecting its processes. As a high school student, I am inspired by the challenge and look forward to contributing to the field of volcanology and disaster management in the future.。
能源危机英语作文
Energy crises have been a significant concern for the world in recent years,affecting economies,societies,and the environment.The following essay will explore the causes of energy crises,their impacts,and potential solutions.Causes of Energy Crises1.Depletion of Fossil Fuels:The primary cause of energy crises is the overreliance on nonrenewable energy sources such as coal,oil,and natural gas.These resources are finite and their extraction and consumption have been increasing at an unsustainable rate.2.Rapid Industrialization and Population Growth:The rapid industrialization of developing countries and the growth of the global population have led to a surge in energy demand,outpacing the supply of traditional energy sources.3.Political Instability and Geopolitical Tensions:Many oilrich regions are politically unstable,leading to disruptions in the supply chain.Geopolitical tensions can also lead to trade restrictions,exacerbating the energy crisis.ck of Investment in Renewable Energy:Insufficient investment in renewable energy technologies has slowed the transition to cleaner,more sustainable energy sources.Impacts of Energy Crises1.Economic Impact:Energy crises can lead to increased energy prices,which can have a ripple effect on the economy,causing inflation and affecting industries that rely heavily on energy.2.Social Impact:High energy prices can disproportionately affect lowerincome households,leading to a decrease in the standard of living and potentially causing social unrest.3.Environmental Impact:The overuse of fossil fuels contributes to environmental degradation,including air pollution,climate change,and the destruction of ecosystems.4.Energy Security:Energy crises can threaten the energy security of nations,making them vulnerable to supply disruptions and price volatility.Potential Solutions1.Investment in Renewable Energy:Increasing investment in renewable energy sourcessuch as solar,wind,and hydroelectric power can help diversify the energy mix and reduce reliance on fossil fuels.2.Energy Efficiency:Improving energy efficiency in buildings,transportation,and industrial processes can significantly reduce energy demand.3.Research and Development:Supporting research and development in new energy technologies can lead to breakthroughs that make renewable energy more viable and costeffective.4.Policy and Regulation:Governments can implement policies and regulations that encourage the use of renewable energy and discourage the use of fossil fuels,such as carbon pricing and subsidies for renewable energy projects.5.International Cooperation:International cooperation is crucial for sharing technology, resources,and best practices to address the global nature of energy crises.In conclusion,energy crises are complex issues that require a multifaceted approach to address.By understanding the causes and impacts,and by implementing a combination of technological,economic,and policy solutions,we can work towards a more sustainable and secure energy future.。
学术英语原文2单元
Unit 2 Energy in TransitionThe era of cheap and convenient sources of energy is coming to an end.A transition to more expensive but less polluting sources must now be managed.John P. HoldrenUnderstanding this transition requires a look at the two-sided connection between energy and human well-being. Energy contributes positively to well-being by providing such consumer services as heating and lighting as well as serving as a necessary input to economic production.But the costs of energy －including not only the money and other resources devoted to obtaining and exploiting it but also environmental and sociopolitical impacts －detract from well-being.For most of human history, the dominant concerns about energy have centered on the benefit side of the energy －well-being equation. Inadequacy of energy resources or (more often) of the technologies and organizations for harvesting, converting, and distributing those resources has meant insufficient energy benefits and hence inconvenience, deprivation and co nstraints on growth. The 1970’s, then, represented a turning point. After decades of constancy or decline in monetary costs －and of relegation of environmental and sociopolitical costs to secondary status －energy was seen to be getting costlier in all respects. It began to be plausible that excessive energy costs could pose threats on a par with those of insufficient supply. It also became possible to think thatexpanding some forms of energy supply could create costs exceeding the benefits.The crucial q uestion at the beginning of the 1990’s is whether the trend that began in the 1970’s will prove to be temporary or permanent. Is the era of cheap energy really over, or will a combination of new resources, new technology and changing geopolitics bring it back? One key determinant of the answer is the staggering scale ofenergy demand brought forth by 100 years of unprecedented population growth, coupled with an equally remarkable growth in per capita demand of industrial energy forms. It entailed the use of dirty coal as well as clean; undersea oil as well as terrestrial; deep gas as well as shallow; mediocre hydroelectric sites as well as good ones; and deforestation as well as sustainable fuelwood harvesting.Except for the huge pool of oil underlying the Middle East, the cheapest oil and gas are already gone. Even if a few more giant oil fields are discovered, they will make little difference against consumption on today’s scale. Oil and gas will have to come increasingly, for most countries, from deeper in the earth and from imports whose reliability and affordability cannot be guaranteed.There are a variety of other energy resources that are more abundant than oil and gas. Coal, solar energy, and fission and fusion fuels are the most important ones. But they all require elaborate and expensivetransformation into electricity or liquid fuels in order to meet society’s needs. None has very good prospects for delivering large quantities of electricity at costs comparable to those of the cheap coal-fired and hydropower plants of the 1960’s. It appears, then, that expensive energy is a permanent condition, even without allowing for its environmental costs.The capacity of the environment to absorb the effluents and other impacts of energy technologies is itself a finite resource. The finitude is manifested in two basic types of environmental costs. External costs are those imposed by environmental disruptions on society but not reflected in the monetary accounts of the buyers and sellers of the energy. “Internalized costs” are increases in monetary costs imposed by measures, such as pollution-control devices, aimed at reducing the external costs.Both types of environmental costs have been rising for several reasons. First, the declining quality of fuel deposits and energy-conversion sites to which society must now turn means more material must be moved or processed, bigger facilities must be constructed and longer distances must be traversed. Second, the growing magnitude of effluents from energy systems has led to saturation of the environment’s capacity to absorb such effluents without disruption. Third, the monetary costs of controlling pollution tend to increase with the percentage of pollutants removed.Despite these expenditures, the remaining uninternalized environmental costs have been substantial and in many cases are growing.Those of greatest concern are the risk of death or disease as a result of emissions or accidents at energy facilities and the impact of energy supplied on the global ecosystem and on international relations.The impacts of energy technologies on public health and safety are difficult to pin down with much confidence. In the case of air pollution from fossil fuels, in which the dominant threat to public health is thought to be particulates formed from sulfur dioxide emissions, a consensus on the number of deaths caused by exposure has proved impossible. Widely differing estimates result from different assumptions about fuel compositions, air pollution control technology, power-plant sitting in relation to population distribution, meteorological conditions affecting sulfate formation, and, above all, the relation between sulfate concentrations and disease.Large uncertainties also apply to the health and safety impacts of nuclear fission. In this case, differing estimates result in part from differences among sites and reactor types, in part from uncertainties about emissions from fuel-cycle steps that are not yet fully operational (especially fuel reprocessing and management of uranium-mill tailings) and in part from different assumptions about the effects of exposure to low-dose radiation. The biggest uncertainties, however, relate to the probabilities and consequences of large accidents at reactors, at reprocessing plants and in the transport of wastes.Altogether, the ranges of estimated hazards to public health from both coal-fired and nuclear-power plants are so wide as to extend from negligible to substantial in comparison with other risks to the population. There is little basis, in these ranges, for preferring one of these energy sources over the other. For both, the very size of the uncertainty is itself a significant liability.Often neglected, but no less important, is the public health menace from traditional fuels widely used for cooking and water heating in the developing world. Perhaps 80 percent of global exposure to particulate air pollution occurs indoors in developing countries, where the smoke from primitive stoves is heavily laden with dangerous hydrocarbons. A disproportionate share of this burden is borne, moreover, by women (who do the cooking) and small children (who indoors with their mothers).The ecological threats posed by energy supply are even harder to quantify than the threats to human health and safety from effluents and accidents. Nevertheless, enough is known to suggest they portend even larger damage to human well-being. This damage potential arises from the combination of two circumstances.First, civilization depends heavily on services provided by ecological and geophysical processes such as building and fertilizing soil, regulating water supply, controlling pests and pathogens, and maintaining a tolerable climate; yet it lacks the knowledge and the resources to replace nature’sservices with technology. Second, human activities are now clearly capable of disrupting globally the processes that provide these services. Energy supply, both industrial and traditional, is responsible for a striking share of the environmental impacts of human activity. The environmental transition of the past 100 years －driven above all by a 20-fold increase in fossil-fuel use and augmented by a tripling in the use of traditional energy forms －has amounted to no less than the emergence of civilization as a global ecological and geochemical force.Of all environmental problems, the most threatening, and in many respects the most intractable, is global climate change. And the greenhouse gases most responsible for the danger of rapid climate change come largely from human endeavors too massive, widespread and central to the functioning of our societies to be easily altered: carbon dioxide (CO2) from deforestation and the combustion of fossil fuels; methane from rice paddies, cattle gusts and the exploitation of oil and natural gas; and nitrous oxides from fuel combustions and fertilizer use.The only other external cost that might match the devastating impact of global climate change is the risk of causing or aggravating large-scale military conflict. One such threat is the potential for conflict over access to petroleum resources. Another threat is the link between nuclear energy and the spread of nuclear weapons. The issue is hardly less complex and controversial than the link between CO2 and climate; many analysts,including me, think it is threatening indeed.能源资源价格低廉、使用便捷的时代已经过去了，目前应向尽管价格较高、但污染较小的资源转变。
能源科普英文作文高中
能源科普英文作文高中英文：Energy is an essential part of our daily lives. It powers our homes, businesses, and transportation. However, not all energy sources are created equal. Some are renewable, while others are non-renewable. In this article, I will discuss the different types of energy sources and their impact on the environment.Renewable energy sources are those that can be replenished naturally and sustainably. Examples include solar, wind, hydro, geothermal, and biomass. These sources are considered clean because they do not emit greenhouse gases or other pollutants. They also have a lower environmental impact than non-renewable sources. For instance, wind turbines and solar panels can be installed on existing structures, such as rooftops, withoutdisrupting the natural landscape. Additionally, hydroelectric dams can provide clean energy withoutemitting harmful pollutants.Non-renewable energy sources, on the other hand, are those that cannot be replenished naturally or sustainably. Examples include coal, oil, and natural gas. These sources are considered dirty because they emit greenhouse gases and other pollutants. They also have a higher environmental impact than renewable sources. For instance, coal mining can lead to deforestation, soil erosion, and water pollution. Oil spills can devastate marine ecosystems and harm wildlife.In conclusion, the type of energy source we use has a significant impact on the environment. Renewable sources are cleaner and have a lower environmental impact than non-renewable sources. It is important that we prioritize the use of renewable energy sources to reduce our carbon footprint and protect the planet.中文：能源是我们日常生活中不可或缺的一部分。
能源学术英语综合教程2
能源学术英语综合教程2Title: Comprehensive Approach to Energy Studies in Academic English.Energy, a crucial aspect of modern life, plays apivotal role in driving economic growth, societal development, and technological advancements. The field of energy studies, therefore, encompasses a vast array of disciplines, ranging from physics and chemistry to engineering and economics. In the academic context, a comprehensive approach to energy studies is essential for fostering a deep understanding of this multifaceted domain. This article aims to explore the intricacies of energy studies through an academic English lens, emphasizing the importance of an interdisciplinary framework.Firstly, it is important to recognize that energy studies are inherently interdisciplinary. Energy systems involve a range of components, including fuel sources, conversion technologies, transmission and distributioninfrastructure, and end-use applications. Each of these components requires expertise from different fields to understand and optimize. For instance, the efficient extraction of fossil fuels requires geologists and petroleum engineers, while the development of renewable energy sources like solar and wind power relies on physicists and materials scientists. Similarly, the economic analysis of energy markets and policies demands the skills of economists and finance experts.In academic English, the language used to communicate ideas and concepts in energy studies must be precise and clear. Technical terms and jargon are common in this field, and it is crucial for students and researchers to master the terminology to ensure effective communication. Additionally, the ability to read and interpret scientific literature, technical reports, and policy documents is paramount. This requires a strong command of the language, including vocabulary, grammar, and sentence structure.Moreover, a comprehensive approach to energy studies in academic English demands critical thinking and analyticalskills. Students must be able to analyze complex energy systems, identify key issues and challenges, and propose solutions. This involves a rigorous evaluation of data, evidence, and arguments, as well as the ability to synthesize information from different sources.Furthermore, the field of energy studies is constantly evolving, with new technologies, policies, and market dynamics emerging constantly. It is, therefore, essential for students and researchers to stay updated with thelatest developments. Academic English, being the lingua franca of international communication, plays a crucial role in disseminating this knowledge across borders. Through seminars, conferences, and published research, ideas and innovations in energy studies are shared globally, driving progress in this crucial area.In conclusion, a comprehensive approach to energy studies in academic English is crucial for fostering a deep understanding of this multifaceted domain. It involves an interdisciplinary framework, precise and clear communication, critical thinking and analytical skills, aswell as staying updated with the latest developments. By cultivating these skills and competencies, students and researchers can contribute effectively to the field of energy studies, driving innovation and progress in this vital area.。
节能智慧之选的英语作文
Energy conservation is a crucial aspect of modern society,as the world grapples with the challenges of climate change,resource depletion,and the need for sustainable development.The concept of energysaving wisdom encompasses a variety of practices and technologies that aim to reduce energy consumption while maintaining or even enhancing the quality of life.Firstly,energyefficient appliances are a cornerstone of energysaving wisdom.By choosing appliances that have a high energy efficiency rating,consumers can significantly reduce their electricity bills and carbon footprint.For instance,LED lights consume less power than traditional incandescent bulbs and last much longer,making them a wise investment for both the environment and the wallet.Secondly,smart home systems are becoming increasingly popular as a means of conserving energy.These systems allow homeowners to monitor and control their energy usage remotely,ensuring that lights and appliances are only used when necessary.For example,a smart thermostat can learn a households temperature preferences and adjust the heating or cooling accordingly,reducing energy waste.Thirdly,renewable energy sources are a key component of energysaving wisdom.Solar panels,wind turbines,and other forms of renewable energy generation can help to reduce reliance on fossil fuels,which are not only finite but also contribute to greenhouse gas emissions.By harnessing the power of the sun,wind,and water,we can create a cleaner, more sustainable energy future.In addition to these technological solutions,energysaving wisdom also involves behavioral changes.Simple actions such as turning off lights when leaving a room,using natural light whenever possible,and unplugging electronics when they are not in use can make a significant difference in energy consumption.Moreover,energysaving wisdom can be applied to transportation as well.Opting for public transport,carpooling,cycling,or walking instead of driving alone can help to reduce fuel consumption and carbon emissions.Electric vehicles and hybrid cars are also becoming more popular as they offer a cleaner alternative to traditional gasolinepowered vehicles.In the realm of construction,energyefficient building designs and materials are essential for reducing energy consumption.Insulation,doubleglazed windows,and energyefficient heating and cooling systems can all contribute to a buildings energy performance.Lastly,education and awareness are vital in promoting energysaving wisdom.Byunderstanding the importance of energy conservation and the benefits it brings, individuals and communities can make informed choices and take action to reduce their energy footprint.In conclusion,energysaving wisdom is a multifaceted approach that combines technology, behavior,and education to create a more sustainable and energyefficient world.By embracing these practices,we can work towards a future where energy resources are used responsibly and the environment is protected for generations to come.。
写一篇如何解决能源短缺问题的英文作文
Addressing Energy Shortages: A Multifaceted ApproachIn the face of mounting global challenges, energy shortages have emerged as a pressing concern that demands immediate attention and innovative solutions. The consequences of inadequate energy supplies extend far and wide, impacting economic growth, social welfare, and environmental sustainability. To effectively tackle this issue, a multifaceted approach is necessary, encompassing diversification of energy sources, promotion of energy efficiency, investment in renewable technologies, and fostering international cooperation.Firstly, diversifying our energy mix is crucial. Reliance on a single or few energy sources, particularly fossil fuels, exacerbates vulnerability to supply disruptions and price volatility. By exploring and developing alternative energy resources such as nuclear, solar, wind, and hydroelectric power, we can create a more resilient energy system. This diversification not only reduces the risk of shortages but also aligns with the goal of transitioning to a low-carbon economy.Secondly, promoting energy efficiency is a high-impact strategy. Enhancing the efficiency of energy use across sectors—from industrial processes to household appliances—can significantly reduce demand and alleviate pressure on energy supplies. Governments can play a pivotal role by implementing policies that encourage energy-efficient practices and technologies, such as providing subsidies for green building projects or setting mandatory efficiency standards for products.Thirdly, investing in renewable energy technologies is paramount. Renewables offer a sustainable and scalable solution to energy shortages. Advancements in solar panels, wind turbines, and battery storage technologies have made renewable energy more competitive and accessible. Governments and private sectors must collaborate to scale up research and development, facilitate financing mechanisms, and streamline regulatory processes to accelerate the deployment of renewable energy projects.Furthermore, fostering international cooperation is vital. Energy shortages are a global challenge that requires collective action. Countries can collaborate on cross-border energy infrastructure projects, share best practices in energy management, and coordinate efforts to mitigate climate change, which indirectly contributes to energy security by promoting sustainable energy sources.Lastly, public awareness and engagement are essential components of any comprehensive strategy. Educating citizens about energy conservation, the benefits of renewable energy, and the importance of energy security can inspire individual actions that, when multiplied across populations, lead to substantial impacts.In conclusion, addressing energy shortages necessitates a comprehensive andintegrated approach that combines diversification of energy sources, promotion of energy efficiency, investment in renewable technologies, international cooperation, and public engagement. By pursuing these strategies in concert, we can pave the way for a more secure, sustainable, and prosperous energy future.。
碳中和的关键不确定因素
碳中和的关键不确定因素英文回答：One of the key uncertainties in achieving carbon neutrality is the availability and scalability of clean energy technologies. While there are already renewable energy sources like solar and wind power, theirintermittent nature and limited capacity pose challengesfor widespread adoption. For example, solar panels only generate electricity when the sun is shining, and wind turbines only produce power when the wind is blowing at the right speed. This means that energy storage solutions and grid infrastructure upgrades are necessary to ensure a reliable and continuous supply of clean energy.Another uncertainty is the cost of transitioning to carbon-neutral technologies. While renewable energy costs have been steadily declining, there are still significant upfront investments required to build the necessary infrastructure. For instance, building a wind farm orinstalling solar panels can be expensive, and the costs may vary depending on factors like location, availability of resources, and government policies. Additionally, retrofitting existing industries and transportation systems to reduce emissions can also be costly and require significant financial resources.Furthermore, the political and regulatory landscape can greatly influence the pace and effectiveness of carbon neutrality efforts. Government policies and incentives play a crucial role in driving the transition to clean energy and reducing carbon emissions. However, politicalpriorities may change over time, leading to shifts in policies and regulations that can either support or hinder carbon neutrality goals. For example, changes in government leadership or shifts in public opinion can result in policy reversals or delays in implementing carbon reduction measures.In addition, technological advancements and innovation are essential for achieving carbon neutrality. While there are already existing clean technologies, further researchand development are needed to improve their efficiency and reduce costs. Breakthroughs in areas such as energy storage, carbon capture and storage, and sustainable transportation can significantly accelerate the transition to a carbon-neutral future. However, the timeline and success of these advancements are uncertain and depend on various factorslike funding, collaboration between researchers and industry, and market demand.中文回答：在实现碳中和过程中，一个关键的不确定因素是清洁能源技术的可用性和可扩展性。
能源意识英文作文
能源意识英文作文Energy consciousness is crucial in today's world. We need to be aware of the energy we use and how it impactsthe environment. It's important to conserve energy whenever possible, whether it's by turning off lights when leaving a room or using energy-efficient appliances.We should also consider using renewable energy sources, such as solar or wind power, to reduce our reliance onfossil fuels. This not only helps to protect the environment but also ensures a more sustainable energyfuture for generations to come.In addition to being mindful of the energy we use at home, it's also important to consider our energy consumption when we're out and about. This could mean using public transportation, carpooling, or even walking orbiking instead of driving. By reducing our reliance on cars, we can help to decrease air pollution and conserve energy.Another important aspect of energy consciousness is being aware of the impact of our energy usage on the global climate. Climate change is a real and pressing issue, and it's crucial that we take steps to reduce our carbon footprint. This could involve making small changes in our daily lives, such as using less hot water, unplugging electronics when they're not in use, or using energy-efficient light bulbs.It's also important to advocate for energy-conscious policies and practices in our communities and beyond. This could involve supporting renewable energy initiatives, advocating for energy-efficient building standards, or encouraging businesses to adopt sustainable energy practices.Overall, energy consciousness is about being mindful of the energy we use and its impact on the environment. By making small changes in our daily lives and advocating for sustainable energy practices, we can all play a role in creating a more energy-conscious world.。
剑桥as物理中的energy
剑桥as物理中的energyEnergy is a fundamental concept in physics, playing a crucial role in various phenomena and processes. In the context of Cambridge AS Physics, the study of energy is essential for understanding the behavior of physical systems and explaining the interactions between different forms of energy.One of the key principles regarding energy is the conservation of energy, which states that the total energy in a closed system remains constant over time. This principle is based on the law of energy conservation, which asserts that energy cannot be created or destroyed, only transferred or converted from one form to another. This concept is central to the study of energy in Cambridge AS Physics, as it allows us to analyze the energy transformations that occur in different processes.In Cambridge AS Physics, energy is classified into various forms, including kinetic energy, potential energy, thermal energy, and electromagnetic energy. Kinetic energy is the energy associated with an object's motion, whilepotential energy is the energy stored in an object due to its position or state. Thermal energy is the energy associated with the motion of particles in a substance, while electromagnetic energy is the energy carried by electromagnetic waves.Understanding how energy is transferred and transformed is crucial for solving problems in physics and explaining the behavior of physical systems. In Cambridge AS Physics, students learn about different energy transfer mechanisms,such as work, heat, and energy conservation. By mastering these concepts, students can analyze complex systems and predict their behavior based on energy principles.In conclusion, the study of energy in Cambridge AS Physics is essential for understanding the fundamental principles of physics and explaining the behavior of physical systems. By mastering the concepts of energy transfer and transformation, students can solve complex problems and gain a deeper insight into the workings of the universe.。
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a r X i v :n u c l -t h /0603066v 3 22 A u g 2006Collisional Energy Loss in a Finite Size QCD MatterMagdalena DjordjevicDept.Physics,Ohio State University,191West WoodruffAvenue,Columbus,OH 43210,USAFebruary 5,2008AbstractComputation of collisional energy loss in a finite size QCD medium has become crucial to obtain reliable predictions for jet quenching in ultra-relativistic heavy ion collisions.We here compute this energy loss up to the zeroth order in opacity.Our approach consistently treats both soft and hard contributions to the collisional energy loss.Consequently,it removes the unphysical energy gain in a region of lower momenta obtained by previous computations.Most importantly,we show that for characteristic QCD medium scales,finite size effects on the collisional energy loss are not significant.1IntroductionThe suppression pattern of high transverse momentum hadrons is a powerful tool to map out the density of a QCD plasma created in ultra-relativistic heavy ion collisions (URHIC)[1]-[3].This sup-pression (called jet quenching)is considered to be mainly a consequence of medium induced radiative energy loss of high energy partons [4]-[7].However,recent non-photonic single electron data [8,9](which present an indirect probe of heavy quark energy loss)showed large disagreement with the ra-diative energy loss predictions,as long as realistic values of parameters are assumed [10].This raised the question of what is the cause for the observed discrepancy.Recent studies [11,12]suggested that one of the basic assumptions that pQCD collisional energy loss is negligible compared to radiative [13]may be incorrect.In [11,12]it was shown that,for a range of parameters relevant for RHIC,radiative and collisional energy losses for heavy quarks were in fact comparable to each other,and therefore collisional energy loss can not be neglected in the computation of jet quenching.This result came as a surprise because from the earlier estimates [13]-[18],the typical collisional energy loss was erroneously considered to be small compared to the radiative energy loss.In [19]it was consequently suggested that the inclusion of collisional energy loss may help in solving the “single electron puzzle”.However,in that study (as well as [11]-[18])finite size effects were not taken into account.A recent paper by Peigne et al.[20]is the first study that made an attempt to include finite size effects in the collisional energy loss.This work suggested that collisional energy loss is large only in the ideal infinite medium case,while finite size effects lead to a significant reduction of the collisional energy loss.However,this paper did not completely separate collisional and radiative energy loss effects.Consequently,it remained unclear how important are the finite size effects on the collisional energy loss.Therefore,it became necessary to consistently compute (only)the collisional energy loss in finite size QCD medium.Additionally,this paper aims to address whether -and to what extent-there is an over-counting between collisional and radiative energy loss computations.The outline of the paper is as follows:In Section 2,we will compute the collisional energy loss in a finite size QCD medium.In Section 3,we will consider the special case when a particle is produced at x 0=−∞(infinite medium case).We will show that in special limits,our calculations recover previous results[14,15].However,contrary to [14,15],our computation does not encounter unphysical energy gain in the low momentum region [14,15].In Section 4we will give a numerical study of the collisional energy loss in both finite and infinite QCD medium.Contrary to the results obtained in Peigne et al.[20]we will show that finite size effects do not have a significant effect on the collisional energy loss.The conclusions and outlook are given in Section 5.2Collisional energy loss in finite size QCD mediumIn this Section we will compute the collisional energy loss (up to the 0th order in opacity)when the jet is produced in a finite size dielectric medium.The contribution to this energy loss comes from one Hard-Thermal Loop (HTL)gluon propagator (see Appendix A),which is the reason why we call it the 0th order in opacity collisional energy loss (note the analogy with the 0th order in opacity radiative energy loss [21]-[23],which is further discussed in Appendix A).In this computation we use the most intuitive approach,i.e.we compute the diagram |M el |shown in Fig.1.Note that the “blob”represents the effective gluon propagator.A proof of the validity of this approach is given in Appendix A.This approach has already been used without proof in [24,25],under the justification that it reproduces the same results as the imaginary time formalism.Figure 1:Feynman diagram for the amplitude that contributes to the collisional energy loss in QCD medium.The large dashed circle (“blob”)represents the effective gluon propagator [21].Similarly as in [23]we introduce the finite size medium by starting from the approach described in [26].We consider a static medium of size L ,and assume that collisional energy loss can occur only inside the medium.The Feynman diagram |M el |(see Fig.1)then represents the source J ,which at time x 0produces an off-shell jet with momentum p ,and subsequently (at x 1>x 0)exchanges a virtual gluon with parton in the medium with momentum k .The jet and the medium parton emerge with momentum p ′and k ′respectively.Since our focus is on heavy quark jets with mass M ,we here neglect the thermal shifts of the heavy quark mass.We assume,as in [27],that J changes slowly with p ′,i.e.that J (p ′+q )≈J (p ).Since we consider both soft and hard contributions,we take into account spin effects.The computation that we present in this paper is gauge invariant [24],but for simplicity we further use Coulomb gauge.The effective gluon propagator shown in Fig.1has both transverse and longitudinal contribu-tions [28]-[32].In Coulomb gauge the gluon propagator has the fallowing form:D µν(ω, q )=−P µν∆T (ω, q )−Q µν∆L (ω, q ),(1)where q =(ω, q )is the 4-momentum of the gluon,while ∆T and ∆L are effective transverse andlongitudinal gluon propagators given by [32]:∆−1T=ω2− q 2−µ22 q 2(1+ωω+| q ||),(2)∆−1L =q 2+µ2(1+ωω+| q ||),(3)where µ2D =g 2T 2(1+N f| q |2,(4)Q 00=1.(5)The matrix element for this 0th order in opacity collisional process can then be written in thefollowing form (for simplicity we here omit color factors,whose contribution we will add in the end)iM el =d 4x 0J (x 0)d 4x 1(−i )d 3p (2π)4D µν(q )e −iq (x 2−x 1)¯u (k ′,λ′)e ik ′x 2igγνu (k,λ)e −ikx 2.(6)Here p ,s ,k and λare the 4-momenta and spins of the incoming jet and medium parton,while thecorresponding primed variables correspond to outgoing jet and medium parton (the medium parton can be quark,antiquark or gluon).The medium partons are considered to be massless,i.e.4-momentumk (k ′)is assumed to be k =(| k |, k )(k ′=(| k′|, k ′)).The condition that the interaction between jet and medium parton has to occur inside the QCD medium of finite size L is implemented through the second θfunction giving maximal interaction time of (t 1−t 0)max =L/v .We will further define x ≡x 1−x 0=(t, x ).The Eq.(6)simplifies toiM el =g2d 3 p (2π)4d 4x 0J (x 0)ei (p ′+q )xd 3xL/vdt e −i (p −p ′−q )x(2π)4δ(k ′−k −q )D µν(q )¯u (p ′,s ′)γµu (p,s )¯u (k ′,λ′)γνu (k,λ)=J (p ′)1E −E ′−ωi M .(7)where E =√√Further,the matrix element given in Eq.(7)has to be squared,averaged over initial spin s of the jet and summed over all other spins.1E2sin[(ω− v· q)L(ω− v· q)212 spins|M|2is given in Appendix B.The differential energy loss is equal to dE el=ωdΓel,where collisional interaction rate dΓel can be extracted from Eq.(9)as(see[27])d3N J dΓel≈1(2π)32E′d3 k(2π)32k′ ξ=q,¯q,g nξeq(k)(1±nξeq(k′)).(10)Hered3N J=d R|J(p′)|2d3p′2 spins|M el|2d3 p′(2π)32k d3 k′E2 d3 k(2π)32k′ωsin[(ω− v· q)L(ω− v· q)212 |M|2and averaging the integrand over the directions of v,we obtain(see Appendix B)∆E el=C R g42J1+|∆T(q)|2(| q|2−ω2)((2| k|+ω)2+| q|2)where J1,J2and J3are given by Eqs.(34-36)in Appendix B,and| q|max is given by the following formula[14]| q|max=Min[E,2|k|(1−| k|/E)dL is computedby assuming that the jet is produced at x0=−∞.The energy loss for afinite size medium is than (simplistically)calculated by multiplying this dE eldL =g42+|∆T(q)|2(| q|2−ω2)((2| k|+ω)2+| q|2)dL =g42+|∆T(q)|2(| q|2−ω2)((2| k|+ω)2+| q|2)dE hardel6v2π3 ∞0n eq(| k|)d| k| | k|/(1+v)| q|∗d| q| v| q|−v| q|ωdω+ | q|max| k|d| q| v| q|| q|−2| k|ωdω|∆L(q)|2(2| k|+ω)2−| q|24| q|4(v2| q|2−ω2) .(18) The soft contribution can be further simplified by keeping only the even contributions in theωintegral(the odd contributions vanish under symmetric integration)dE softel3v2π3 ∞0| k|n eq(| k|)d| k| | q|∗0d| q|v| q|−v| q|ω2dω |∆L(q)|2+1| q|2)(v2−ω26πv2µ2D | q|∗0d| q| v| q|−v| q|ω2dω |∆L(q)|2+1| q|2)(v2−ω22(1+N f| q|2and|∆T(q)|→1dL =g4| q|2v| q|−v| q|ωdω+ | q|max2| k|/(1+v)d| q| 4| q|2−v22| q|2| q|2−(1−v2)| k|(| k|+ω)4Numerical resultsIn this section we give a numerical study of the collisional energy loss in a QCD medium as presented in Sections 3and 4.To do this,we further assume that the QCD plasma is characterized by T =0.225GeV,N f =2.5and α=0.3.For the light quark mass we take M =µD /√6)≈0.5GeV is the Debye mass.The charm mass is taken to be M =1.2GeV,and thebottom mass is M =4.75GeV.4.1Collisional energy loss in infinite QCD medium51015202530p GeV00.20.40.6E el ETGBTLIGHT51015202530p GeV0.10.20.3 0.1E el ETGBTCHARM 051015202530p GeV0.040.080.12E elETGBTBOTTOMFigure 2:Fractional collisional energy loss is shown as a function of momentum for light,charm and bottom quark jets (left,center and right panels respectively).Dash-dotted curves are obtained by using Eq.(16)from this paper.Dashed curves correspond to Eqs.(8)and (12)from [15],while dotted curves are obtained by using Eq.(4.1)from [14].Assumed thickness of the medium is L =5fm.In Fig.2we compare our collisional energy loss results in an infinite QCD medium (Eq.(16))with previous computations by [14,15].We see that,while both BT [15]and TG [14]computations lead to unphysical negative energy loss results in the low momentum region,our computations give positive collisional energy loss in the whole jet momentum range.This is particularly important in the bottom quark case where the unphysical behavior persists up to 5GeV in BT [15]case and up to 2GeV in TG [14]case.The reason for this behavior is that only the leading logarithmic contribution was considered in the final steps of both BT and TG computations.Note that the problem of unphysical energy gain was addressed in Ref.[33],by including fully dressed gluon propagator in their calculations.However,that approach leaded to another problem,since the unphysical momentum scale | q |∗appeared in the collisional energy loss results [33].Therefore,our results present a first complete solution to the unphysical energy gain problem.Our numerical results agree with BT only in the asymptotic regions,which is likely the consequence of the following:1)BT made a sharp (instead of continuous)transition from soft to hard limit and 2)they introduced a sharp boundary in the energy loss computations depending on whether the initial jet energy is much larger/smaller than M 2/T .Despite the fact that the BT computations are more up to date and treat the collisional energy loss more consistently than TG,we see that our results show better agreement with TG [14]computations.Particularly,in case of light and charm quark jets,there is a quite good agreement between our results and those of TG [14].The good agreement is probably because the forward emission only (see Section 3)provides a quite plausible estimate for the collisional energy loss.However,for bottom quarks we see that neither BT nor TG computations provide a reasonable estimate for the collisionalenergy loss.Therefore,in this case,the more accurate computation of collisional energy loss (i.e.our Eq.(16))is needed.4.2Collisional energy loss in a finite QCD medium51015202530p GeV0.10.20.30.4E elELIGHT51015202530p GeV00.050.10.150.2E el ECHARM51015202530p GeV00.020.040.060.08 E elE BOTTOMFigure 3:Fractional collisional energy loss is shown as a function of momentum for light,charm and bottom quark jets (left,center and right panels respectively).Full curves correspond to finite medium case (see Eq.(14)),while dash-dotted curves correspond to infinite medium case (see Eq.(16)).Assumed thickness of the medium is L =5fm.246810L fm0.10.20.30.4 EelEE 10GeVucbFigure 4:Fractional collisional energy loss is shown as a function of thickness of the medium for light,charm and bottom quark jets (upper,middle and lower set of curves respectively).Full curves correspond to finite medium case (see Eq.(14)),while dash-dotted curves correspond to infinite medium case (see Eq.(16)).Initial momentum of the jet is 10GeV.Figures 3and 4show the comparison between collisional energy loss in infinite and finite size QCD medium.Contrary to [20],we find that a finite size medium does not have a large effect on the collisional energy loss.The discrepancy between our results and those presented in [20]is due to the fact that what was called collisional energy loss in [20],is in fact combination of collisional and part of the 0th order radiative energy loss.Actually,the calculation in [20]does not present a complete 0th order energy loss either,since transition radiation [23]was not included in their computation.510L fm0.050.10.150.2 E el ECHARMFigure 5:Fractional collisional energy loss is shown as a function of thickness of the medium for charm quark jet.Dash-dotted curves correspond to infinite medium case (see Eq.(16)),while dashed curve show what would be the collisional energy loss in finite size medium if term 2ωJ 2−J 3=0.Dotted curve shows the extra contribution to the collisional energy loss in finite size medium,due the fact that 2ωJ 2−J 3=0.Initial momentum of the jet is 20GeV.Contrary to naive expectations,from Figs.3and 4we found that collisional energy loss in a finite size medium can be somewhat larger than in an infinite medium.The reason is that in Eq.(16)there exists a term 2ωJ 2−J 3.If this term were equal to zero (as in the case of infinite medium),the energy loss in a finite medium case would always be smaller than in an infinite medium,as naively expected (compare dashed and dot-dashed curves in Fig.5).However,in the finite medium case,the term 2ωJ 2−J 3=0,giving a noticeable positive contribution (see the dotted curve in Fig.5)which will lead to somewhat larger (overall)energy loss in the finite medium case.To further discuss this,let us look at the Eqs.(34-36)(Appendix A)in the finite medium case,and compare them to the corresponding Eqs.(52-54)in the infinite medium.The δfunction in Eqs.(52-54)ensures energy conservation,which is satisfied when the jet is produced at −∞.Consequently,in this case 2ωJ 2−J 3≡0(see Eqs.(53)and 54)).However,when the jet is produced at finite time x 0,time translation invariance is broken,and therefore the energy of the collisional process is not conserved,leading to 2ωJ 2−J 3=0.4.3Comparison between collisional and radiative energy loss in a finite size QCD mediumIn Appendix A we showed how to separate the contributions to the collisional and radiative energy loss.In this section we directly compare these two contributions in the case of finite size QCD medium.To compute the net radiative energy loss,we note that there are three important effects that control this energy loss in a QCD medium.These effects are the Ter-Mikayelian effect [21],transition radiation [23]and medium induced radiation [34].In [23],we combined these effects to compute the net radiative energy loss.We here use these results for the purpose of further comparison with the collisional energy loss.Note that in these computations,in order to simulate confinement in the vacuum,we introduce an effective gluon mass in the vacuum m g,v ≈ΛQCD =0.2GeV (for more details see [21]).51015202530p GeV00.10.20.3E ELIGHT 051015202530p GeV0.050.10.150.2 E ECHARM51015202530p GeV00.020.040.060.08 EE BOTTOM Figure 6:The comparison between collisional and radiative fractional energy loss is shown as a func-tion of momentum for light,charm and bottom quark jets (left,center and right panels respectively).Full curves show the collisional energy loss,while dot-dashed curves show the net radiative energy loss.Assumed thickness of the medium is L =5fm and λ=1.2fm [19].246L fm00.10.20.3E ELIGHT E 10GeV246L fm00.10.20.3E ECHARM E 10GeV246L fm00.040.080.12E EBOTTOME 10GeVFigure 7:The comparison between collisional and radiative fractional energy loss is shown as a function of the thickness of the medium.Light,charm and bottom quark cases are shown on the left,center and right panels respectively.Full curves show the collisional energy loss,while dot-dashed curves show the net radiative energy loss.Mean free path is λ=1.2fm [19].Initial momentum of the jet is E =10GeV.In Figures 6and 7we show the collisional and radiative energy loss as a function of jet energy and thickness of the medium,respectively.We see that collisional energy loss is comparable with the net radiative energy loss in the medium.Therefore,the collisional energy loss contribution is significant and must be included in the computation of jet quenching in a QCD medium.In particular,we note that in the lower momentum (i.e.p <10GeV)range,the collisional energy loss dominates the radiative one.At RHIC,jet suppression is mostly measured in this (lower)mo-mentum range.Therefore,contrary to previous studies [4]-[7],our results indicate that it is collisional instead of radiative energy loss which gives the dominant contribution to the observed suppression values.5ConclusionThis paper addressed the0th order contribution to the collisional energy loss in afinite size QCD medium.The interest in the collisional energy loss has been renewed by the recent studies[11,12], particularly in the context of the single electron puzzle[8]-[10].In Refs.[11,12]it was claimed that,contrary to the previous beliefs,for the parameter ranges relevant in URHIC,radiative and collisional energy loss become comparable.However,a recent study by Peigne et al.[20]suggested that collisional energy loss is large in the ideal infinite medium case,while thefinite size medium effects lead to significant reduction of the collisional energy loss.The paper[20],however,did not completely separate collisional from radiative energy loss.Additionally,even in the infinite medium case,the problem of collisional energy loss was not completely solved.Previous computations obtained unphysical results in the low momentum re-gions[14,15],and an approach to solve this problem[33]leaded to the results dependent on the unphysical momentum scales.In addition,these computations introduced quite a large uncertainty in the heavy quark(especially bottom)collisional energy loss,since they leaded to noticeably different numerical results.The above reasons and the previously discussed single electron puzzle,motivated us to provide a detailed study of the0th order collisional energy loss in afinite size QCD medium created in URHIC.First,in Appendix A we showed that,though0th order collisional and radiative energy loss contributions come from the same one-loop HTL diagram,there is no overlap between collisional and radiative energy loss computations.More specifically,while0th order collisional energy loss comes from the processes which have the same number of incoming and outgoing particles,the radiative energy loss has one gluon more as an outcome of the process.Additionally,we showed that in the0th order calculations,there are no interference effects between collisional and radiative energy loss,which is different from a result in the recent paper[35].The absence of interference effects comes from the fact that,contrary to[35],in our study we consistently treat the gluon dispersion relation in the medium. This leads to the following conditions:1)for the0th order collisional energy loss contributions,the energy of the exchanged(virtual)gluon has to be smaller,or equal,to the gluon momentum,and2) for the radiative energy loss contributions the energy of the radiated gluon has to be larger than its momentum.Therefore,these two contributions take non-zero values in non-overlapping regions,and consequently cannot interfere with each other.In the case of infinite medium,our computation interpolates smoothly between soft to hard contri-butions and,contrary to[15],does not require the introduction of an arbitrary intermediate momentum scale.Additionally,our computation treats the lower momentum region consistently,removing the unphysical energy gain results obtained in previous computations[14,15].In the case offinite size QCD medium,contrary to the study by Peigne et al.[20]we showed thatfinite size effects have small effect on the collisional energy loss.Therefore,consistently with the claims in Refs.[11,12]and our recent single electron suppression estimates[19],we here showed that collisional energy loss is important,and has to be included in the computation of jet quenching. AcknowledgmentsI thank Ulrich Heinz for discussions and critical reading of the manuscript.Valuable discussions with Eric Braaten,Miklos Gyulassy,Yuri Kovchegov and Xin-Nian Wang are gratefully acknowledged.This work is supported by the U.S.Department of Energy,grant DE-FG02-01ER41190.A HTL contribution to the collisional energy lossIn this section we will derive the formula for the lowest order collisional interaction rate.The zerothorder contribution to both radiative and collisional rates comes from the diagram M given in Fig.8.We will below use this diagram as a starting point to separate contributions of collisional and radiative energy loss.in Fig.9.Figure9:Diagram M nThe definition of“black circles”in Fig.9is shown in Fig.10.=++Figure10:Definition of the“black circles”in Diagram M n.The diagram M contains both collisional and radiative0th order contribution to the jet energy loss.It is useful to look at the simple n=1case(see Fig.11)to better understand this.Figure11:n=1contribution to the HTL diagram.The contribution to the collisional energy loss is obtained by“cutting”(i.e.putting on shell)the propagators of parton k′and p′.On the other hand,the radiative contribution is obtained by putting the parton propagator p′and the gluon propagator q on shell.From this,it follows that collisional and radiative contributions come from different diagrams.Furthermore,from the conservation of energy and momentum it can be shown that cutting the gluon propagator q,leads to the condition|ω|>| q|, while cutting the propagator of parton k′leads to the condition|ω|<| q|.Consequently,there is no interference(and over-counting)between collisional and radiative contributions1.The computation of the radiative0th order energy loss has already been a subject of our previous work[23,21].So,the contributionsfrom the diagrams which give raise to the radiative energy loss,will not be further addressed here.As we can see from the right side of the Fig.11,there are two contributions from diagram M1to the collisional rate.These two contributions can be combined into one by allowing that the energy of the gluon can take both positive and negative values.Therefore,the contribution to the collisional rate from diagram M1(dΓM1)is equal tod3N J dΓM1=d3 p′(2π)32kd3 k′1Note that in this paper,we treat only the0th order contribution to collisional energy loss.It is,however,possible that interference effects and/or over-counting between collisional and radiative energy loss contributions would occur in the higher order computations.Higher order contributions are a separate non-trivial problem,which is not considered in this paper.M n is equal tod3N J dΓMn =d3 p′(2π)32kd3 k′(2π)32E′d3 k(2π)32k′n eq(k)∞ n=0n−1 i=0M E i M†E n−1−i=d3 p′(2π)32kd3 k′(2π)32E′d3 k(2π)32k′n eq(k)|M el|2,where(see Fig.14)M el=∞ n=0M E n(25)Figure14:Feynman diagram M el for the collisional energy loss in QCD medium.The large dashed circle(“blob”)represents the effective gluon propagator[21].To prove the above,we willfirst compute|M el|2|M el|2=∞ i=0∞ j=0M E i M†E j=∞ i=0∞ n=i M E i M†E n−i(26)where in the last step we defined n =i +j →j =n −i .Since∞ i =0∞ n =i=∞n =0n i =0,(27)we can conclude thatd 3N J d Γ=d 3 p′(2π)32kd 3 k′E 2d 3 k(2π)32k ′ωsin[(ω− v · q )L (ω− v · q )21e |k |/T −1+N f2spins |M|2.M =g 2D µν(q )¯u (p ′,s ′)γµu (p,s )¯u (k ′,λ′)γνu (k,λ).(31)In Coulomb gauge,the only non-zero terms in the effective gluon propagator are given in Eqs.(4)and (5),which together with Eqs.(1-3)reduce the Eq.(31)toM =g 2∆L (q )¯u (p ′,s ′)γ0u (p,s )¯u (k ′,λ′)γ0u (k,λ)+g 2∆T (q )(δij −ˆq i ˆq j )¯u (p ′,s ′)γi u (p,s )¯u (k ′,λ′)γj u (k,λ).(32)Here ˆq i ≡q i /| q |.The matrix element given in Eq.(32)has to be squared,averaged over initial spin s of the jet andsummed over all other spins.After evaluating the Dirac traces,and applying the assumption that | q |≪E (highly energetic jet)we obtain similarly to [24]1| q |2)+| k ′|( v · k −v · q q · k | q |2)( v · k′− v · q q · k′| q |2).(33)In a static medium,the collisional energy loss does not depend on the direction of v .Therefore,theEq.(29)can be further simplified by averaging the integrand over the directions of v .The integrals that are required areJ 1=d Ω2v ]24| q |v 2Si ((v | q |+ω)Lv)−1v )v )4πsin[(ω− v · q )L(ω− v · q )2(ω− v · q )=1v)−Ci ((v | q |+ω)Lv | q |−ω)(35)andJ 3=d Ω2v]221−cos(LωL | q |.(36)By using Eqs.(34-36),it can be shown thatd Ω2v ]2| q |−J 2ˆq i4πsin[(ω− v · q )L(ω− v · q )2v i v j =J 1v 2| q |2−ω22| q |2ˆq i ˆq j+2ωJ 2−J 34πsin[(ω− v · q )L(ω− v · q )216g 4E 2|∆L (q )|2(| k || k ′|+ k · k′)=16g 4E 2|∆L (q )|2(2|k |+ω)2−| q |2dΩ2v]2| q|2)+|k′|( v· k− v· q q· k4πsin[(ω− v· q)L(ω− v· q)216g4E2|∆T(q)|2× 2( v· k− v· q q· k| q|2)+(| k|| k′|− k· k′)(v2− v· q q· v4| q|4 (v2| q|2−ω2)J1+2ωJ2−J3 .(41)Therefore,averaging the sin[(ω− v· q)L(ω− v· q)212v]22 spins|M|2 =16g4E2(|∆L(q)|2(2| k|+ω)2−| q|24| q|4 (v2| q|2−ω2)J1+2ωJ2−J3 ).(42) Since the collisional energy loss does not depend on the direction of v,the Eq.(29)can be written as∆E el=C R 1(2π)32kn eq(k) d3 k′2v]22 spins|M|2=C RE2 | k|| k′|d| k|d| k′|d cosθn eq(| k|)sin[(ω− v· q)L(ω− v· q)212| k|| k′|,(44)where| k′|=| k|+ω.We can now introduce the change of variables from| k|,| k′|and cosθ,to| k|,ωand| q|,which reduces the Eq.(29)to the following form∆E el=C RE2 n eq(| k|)d| k|| q|d| q|ωdωsin[(ω− v· q)L(ω− v· q)212π4 n eq(| k|)d| k|| q|d| q|ωdω(|∆L(q)|2(2| k|+ω)2−| q|24| q|4 (v2| q|2−ω2)J1+2ωJ2−J3 ).(45)。