The Higgs Particle希格斯粒子

• The strength of the Higgs coupling is proportional to the mass of the particles involved so its coupling is greatest to the heaviest decay products which have mass < mH/2. For example, if mH > 2Mz then the couplings for decay to the following particle pairs:
Z0Z0 : W+W- : τ+τ- : pp : μ+μ- : e+e-
are in the ratio
1.00 : 0.88 : 0.02 : 0.01 : 0.001 : 5.5 x 10-6
August 22, 2002
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• Mass constraints from self-consistency* of the Standard Model :
Higgs Mechanism III. Searches for the Higgs Particle IV. Future Prospects V. What We Will Learn When We Find It
August 22, 2002
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I. Mass in the Standard Model
1961 – 1968 Glashow, Weinberg and Salam (GWS) developed a theory that unifies the electromagnetic and weak forces into one electroweak force.
Electromagnetic Force – mediator: photon (mass = 0) felt by electrically charged particles
W’s
August 22, 2002
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W+
W-
Z0
The Higgs Field and Higgs Boson
The neutral Higgs field permeates space and all particles acquire mass via their interactions with this field.
• Dominant Production Mechanisms :
LEP:
e+e- H0 Z0
Tevatron:
gg H0
qq H0W or H0Z
August 22, 2002
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Searches at the Large Electron-Positron Collider (LEP) at CERN
August 22, 2002
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Spontaneous Symmetry Breaking
This is a phenomenon that can occur when the symmetries of the equations of motion of a system do not hold for the ground state of the system.
above background
mH 115.6 GeV/c2
August 22, 2002
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mH > 114.1 GeV/c2
Searches at the Tevatron
Search Methods: qq (H0 bb)(W l ν) qq (H0 bb)( Z0 l+l-) (l = e, μ) CDF: also hadronic decays of W,Z Dzero: also Z ν ν Run I: CDF and DZero took 100 pb-1 of data each and no signal seen though cross section limits were set Run II: CDF and DZero expect 10 fb-1 of data each
Final States with Good Sensitivity to Higgs Boson:
1. e+e- (H0bb) (Z0qq)
BR 60%
2. e+e- (H0bb) (Z0νν)
BR 17%
3. e+e- (H0bb) (Z0e+e- , μ+μ-)
BR 6%
Wμ1,2,3, Bμ
W+, W-, Z0, γ
This transformation is the result of a phenomenon known as Spontaneous Symmetry Breaking. In the case of the electroweak force, it is known as the Higgs Mechanism.
4. e+e- (H0τ+τ-) (Z0qq)
5. e+e- (H0qq) (Z0 τ+τ-)
BR 10%
August 22, 2002
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Aerial view of LEP at CERN
Au源自文库ust 22, 2002
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LEP Search Results
August 22, 2002
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IV. Future Prospects
The Large Hadron Collider (LHC):
2019
• pp collider with a center of mass energy of 14 TeV
• ATLAS and CMS detectors optimized for Higgs searches
What is the origin of the particle masses?
August 22, 2002
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Particle Masses (GeV/c2)
Up 0.003 Down 0.006
νe <1 x 10-8 Electron 0.000511
August 22, 2002
The Higgs Boson
• neutral
• scalar boson (spin = 0)
Ho
• mass = ?
August 22, 2002
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III. Searches for the Higgs Particle
What properties are important?
electromagnetic forces become comparable..
August 22, 2002
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GWS Electroweak Theory
The theory begins with four massless mediators for the electroweak force: Wμ1,2,3 and Bμ.
The Higgs Particle
Sarah D. Johnson University of La Verne
August 22, 2019
August 22, 2002
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Outline
I. Mass in the Standard Model II. Electro-Weak Force Unification and the
• Higgs mass range between 100 GeV/c2 and 1TeV/c2
Next Linear Collider:
after 2019
• e+e- collisions at 500+ GeV
• precision measurements of Higgs couplings to a few percent
Charm 1.3
Strange 0.1 νμ
<0.0002 Muon 0.106
Top 175 Bottom 4.3 ντ <0.02 Tau 1.7771
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Photon 0
Gluon 0 Z
91.187 W± 80.4
Questions:
Why is there such a large range of quark masses? Why is there such a large range of lepton masses? Why are the neutrino masses so small?
Why do the W and Z have mass, but the photon and the gluon do not?
August 22, 2002
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II. Electroweak Force Unification and the Higgs Mechanism
130 GeV/c2 < MH < 190 GeV/c2
*The discovery of a Higgs boson with a mass less than 130 GeV/c2 would imply “new physics” below a grand unification (GUT) scale energy of 1016 GeV/c2
In 2000 at LEP2 with a center of mass energy of > 205 GeV:
ALEPH: signal three standard deviations above background with mH 115 GeV/c2
All four experiments: signal reduced to two standard deviations
LEP1: 17 million Z0 decays
mH > 65 GeV/c2
LEP2: 40,000 e+e- W+W- events
e+e- H0Z0 has background from W+W- and Z0Z0 events, but b-tagging and kinematic constraints can reduce these backgrounds.
In the Higgs Mechanism, as the result of choosing the correct gauge, the massless gauge field “eats” the Goldstone bosons and so acquires mass. In addition, a “mass-giving” Higgs field and its accompanying Higgs boson particle emerge.
Weak Force – mediators: W+,W-, Z0(mass ~ 80-90 GeV/c2) felt by quarks and leptons
August 22, 2002
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For two protons in a nucleus the electromagnetic force is 107 times stronger than the weak force, but, at much shorter distances (~10-18 m), the strengths of the weak and the
August 22, 2002
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Higgs Mechanism
Goldstone’s Theorem - The spontaneous breaking of a continuous global symmetry is always accompanied by the appearance of massless scalar particles called Goldstone bosons.