Production of $e^+e^-$ pairs to all orders in $Zalpha$ for collisions of high-energy muons

a r X i v :h e p -p h /9807311v 2 24 J u l 1998UL–NTZ 14/98Production of e +e −pairs to all orders in Zαfor collisions of high-energy muons with heavy nucleiD.Ivanov 1,2∗,E.A.Kuraev 3†,A.Schiller 1‡,and V.G.Serbo 2,4§1Institut f¨u r Theoretische Physik and NTZ,Universit¨a t Leipzig,D-04109Leipzig,Germany2Novosibirsk State University,Novosibirsk,630090Russia 3Joint Institute of Nuclear Research,Dubna,141980Russia4INFN,Via Celoria 16,I-20133Milano,ItalyJuly 24,1998AbstractThe e +e −pair production in collisions of muon with atoms is considered to all orders in the parameter Zα.We obtain energy distribution of e +and e −as well as energy loss of muon passing through matter with heavy atoms.The found corrections to the Born contribution do not depend on the details of the target properties except of a simple factor.For the considered example of Pb target the muon energy loss corrections vary from −65%to −10%depending on the pair energy.1IntroductionThe production of e +e −pairs in collisions of high energy muons with nuclei and atoms is important for a number of problems.In particular,this process is dominant for energy losses of muons passing through matter.An precise knowledge of these losses is necessary for the construction of detectors and µ+µ−colliders and an estimation of shielding at high energy colliders.In Born approximation various cross sections for the process under discussion (A de-notes an atom or a nucleus with charge number Z )µA →µA e +e −(1)have been calculated in Refs.[1]-[5].A recent short review on the muon energy loss at high energy can be found in Sect.23.9of[6].Some useful approximate formulae and figures are given in[7].In all mentioned papers effects of high order corrections in the parameterν=Zα≈ZE2µ,m esame complicated method has been used as in Ref.[8]which basically uses approximated relativistic wave functions of e+and e−in the Coulombfield of the nucleus A.Our approach is more simple and transparent.It is based on cross sections for the virtual processγ∗A→e+e−A(whereγ∗denotes the virtual photon with4-momentum squared q2<0)which has been recently obtained in Ref.[13]by a direct summation of the corresponding Feynman diagrams.Forε±≪Eµour Eqs.(20)and(30)coincide with Eqs.(38)and(39)of[11],respectively,while our Eq.(14)coincides with the corresponding equation of[12]only in the main logarithmic approximation.(It should be noted,however,that the results of[12]for the discussed process are also presented in logarithmic accuracy.)The outline of our paper is as follows:In Sect.2we obtain the energy distributions for electrons and positrons.The next Sect.is devoted to the muon energy loss.In Sect.4we summarise our results and compare the obtained Coulomb corrections with the Born contributions.Additionally we discuss CC for similar processes with other charged projectiles(e,π,p instead ofµ).Our main notations are collected in Fig.1:q andωare the4-momentum and energy of the virtual photon generated by the projectile muon and k1,...,k n are the4-momenta of the photons exchanged with the nucleus.Besides we usex±=ε±Eµ,Q2=−q2>0.(5)Throughout the paper the well known functionf(ν)=ν2∞ n=11 3πZ2α4sections σT and σS have been calculated recently in Ref.[13]1:dσT =dσT 1+dσT2=4m 2e [L −f (ν)]m 4em 2e +Q 2x +x −dx +,dσS=dσS1+dσS 2=4m 2e[L −f (ν)]4m 2e Q 2x 2+x 2−m e−1m 2e−1ω,Qπ(1−y ) 1−Q 2min ωdQ 2π(1−y )1+λ2CdωQ 2(12)whereλ=11−y,Q 2min=y 2E µ.(13)For the considered case of muon projectile C =D =1,other particles are discussed inSect.4.Eqs.(8)-(13)are the basis for our following calculations.Integrating Eq.(8)over Q 2from Q 2min to infinity (the upper limit can be set to infinity due to the fast convergence of the integral)we obtain the known Born contribution and the new expression for CC:dσCoul =−σ0f (ν)F (x,y )dωξ−a +4−a −λm 2ey 21In Eqs.(46),(49)and (59)of [13]a factor x +x −is missing in the integrands of quantities σS1andσS2.Since the integration variables can be transformed as followsdωω2(16)Eq.(14)describes the energy distribution of e+and e−in CC.In the limitξ≪1(or y≪m e/mµ)the function F(x,y)is approximated byF(x,y)=(1+2x2++2x2−)ln1ξ[1−y+y2+2(1−y−y2)x+x−].(18) It is easy to see that the main contribution toσCoul arises from the regionm2e≪ε+ε−≪ m e Eµy ,F(y)=(1−y)F1(z)+y2F2(z),z=m2µ1−y(20)whereF 1(z )=4415−715ln z ++−444+z +3815zln4+4,F 2(z )=−46ln z ++−2z (4+z )+71+41+z z3+8zz+20450+ (22)whereas at large z ≫1it is powerlike suppressed:F (y )=13zln z +6+5zln z +1+1This quantity can be calculated as−1dx=n12m e/Eµydσ4mµ−1.771 (25)and for complete screeningS Born=S0 (1−δ1)ln18912m e19π2m em e 2=0.0352.(27)From Eqs.(20)and(24)we derive the Coulomb correctionS Coul=−f(ν)1F(y)dy=−(1−δ1)f(ν)S0.(28)In the integration we have used as lower limit zero,since the region near the threshold y min=2m e/Eµcan be safely neglected.4DiscussionIn this paper we have presented the Coulomb correction to pair production of high energy muons on atoms.Differently to the Born contributions of various distributions,these corrections do not depend on the target properties,except of a simple factorσ0f(ν)(if we do not consider the exceptional case of pair energies close to the muon energy).To demonstrate the relative importance of CC we discuss two simple examples:Firstly, we present in Fig.3the ratio of the spectral distribution dσCoul/dy to the corresponding Born cross sectiondσCoul/dy12F a(y,Eµ)(29) where the universal function F(y)is given in Eq.(20),and values for F a(y,Eµ)are taken from Table I of[4]for collisions of muons with energy Eµ=86.4GeV on lead target, f(ν)=0.331.The presented ratio varies from about−65%to roughly−10%in the considered interval of pair energies.Secondly,we compare the stopping power S Coul with the Born term in the two limiting cases of Eqs.(25)and(26).For muon scattering on a Pb target S Coul/S Born=−15%at Eµ=25GeV without screening and−7.7%for the case of complete screening.Figure3:Ratio of Coulomb to Born energy distribution vs.energy fraction y for muon collisions on Pb target at Eµ=86.4GeVIt might be useful to present a simple expression for the contribution toσCoul above some cutω>ωcut where this cut is restricted to the region2m e≪ωcut≪m e Eµ/mµ. From Eq.(20)we obtainσCoul(ωcut)=−721l+101mµωcut.(30)The expression(20)does not remain valid close to the thresholdωmin=2m e.Therefore, from Eq.(30)the Coulomb correction to the total pair production cross section can be obtained only in leading logarithmic approximation choosingωcut=2m e:σCoul=−28m2ef(ν) ln Eµthese changes are essential only for y close to1where we should take into account the nucleus form factor,too.For the proton case the nucleus form factor becomes important for somewhat smaller y where the influence of the proton form factors is still small. AcknowledgementsV.G.S.is very grateful to F.Palombo for useful discussions and warm hospitality at INFN.E.A.K.is supported by INTAS grant93-239ext.V.G.S.acknowledges the fellowship by the Italian Ministry of Foreign Affairs and support from Volkswagen Stiftung(Az.No. 1/72302).D.I.acknowledges support from the German BMBF under grant Nr.057LP91 P0.References[1]ndau and E.M.Lifshitz,Sov.Phys.6,244(1934).[2]G.Racah,Nuovo Cim.14,93(1937).[3]S.R.Kel’ner,Sov.J.Nucl.Phys.5,778(1967).[4]S.R.Kel’ner and Yu.D.Kotov,Sov.J.Nucl.Phys.7,237(1968).[5]A.I.Nikishov,Sov.J.Nucl.Phys.27,677(1978).[6]R.M.Barnett et al.,Phys.Rev.D54,1(1996)and1997off-year partial update forthe1998edition available on the PDG WWW pages(URL:/).[7]A.Van Ginneken,Nucl.Instr.and Meth.A251,21(1986).[8]H.A.Bethe,L.C.Maximon,Phys.Rev.93,768(1954);H.Davies,H.A.Bethe,L.C.Maximon,Phys.Rev.93,788(1954).[9]A.I.Akhiezer,V.B.Berestetskii:Quantum electrodynamics(Nauka,Moscow1981).[10]V.B.Berestetskii,E.M.Lifshitz,L.B.Pitaevskii,Quantum Electrodynamics(Nauka,Moscow,1989).[11]A.I.Nikishov,N.V.Pichkurov,Yad.Fiz.35,964(1982)[Sov.J.Nucl.Phys.35,561(1982)].[12]C.A.Bertulani,G.Baur,Phys.Rep.163,299(1988),Sect.7.3.[13]D.Ivanov,K.Melnikov,Phys.Rev.D57,4025(1998).[14]V.M.Budnev,I.F.Ginzburg,G.V.Meledin,V.G.Serbo,Phys.Rep.C15,181(1975).[15]V.N.Baier,V.S.Fadin,V.A.Khoze,E.A.Kuraev,Phys.Rep.78,293(1981).。