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Higgs Mechanism
13.9.2007 - Cyril Topfel
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What to expect from this Presentation (Table of Contents)
Some very limited theory explanation
Higgs at LEP Higgs at the Tevatron Higgs at the LHC Conclusions
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The Higgs-Mechanism ... is responsible for the mass of massive particles
... causes the electroweak symmetry breaking
... is the last piece of the Standard Model that hasn‘t been shown experimentally
The Higgs particle will be found at the LHC, if it exists.
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Some TheoryHiggs-Potential
Symmetric, but non-symmetric in the ground state.
v = 246 GeV
Non-zero vacuum expectation value:
GF = 1.166 e-5 GEV-2
No constrain for In the Higgs mass
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Some TheoryMasses of the gauge bosons through symmetry breaking:
Due to gauge invariance
Fermions get their masses through Yukawa coupling with theHiggs field. The coupling strengths are free parameters. (But gtop~1, why?
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Higgs Production at e+e- Colliders
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Higgs Production at Hadron Colliders
gg Fusion
W/Z Fusion
H-radiation
10x >
> (~= at low mass)
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Higgs Decay ModesH mass Dom. proc
<130 GeV H->bb (ff)
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>130 GeV H->V*V #
(H)
~10MeV
<GeV
#: V*V: one real and one virtual Vector Boson: W*W or Z*Z
>180 GeV H->VV
tt-channel not very relevant m(H)>500 -> ~ m
~10MeV
H->: rare decays, but clean signature
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Higgs at LEP LEP1 sensitive to:
e+e- -> (H0 -> bb) (Z0->) e+e- -> (H0 -> bb) (Z0->l+l-) (l: e or
)
LEP2 additionally involved decay modes
e+e- -> (H0 -> bb) (Z0->qq) e+e- -> (H0 -> ) (Z0->qq) e+e- -> (H0 -> qq) (Z0->)
ETmiss !
leptonic !
BR*
* @ m(H) = 115 GeV
17%
6%
60%
10%Background!
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Higgs @ LEP -> 3 sigma @ 115 GeV!
Q = L(s+b)/L(b)
BG (simulated)
S (simulated)
1-sigma (BG)
2-sigma (BG)
BUT: Combined data only 1.7 sigma95% CL of Higgs mass lower bound of 114 GeV
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Higgs @ Tevatron
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Constrains on Higgs mass
m(H)>114 GeV (LEP II) m(H)<166 GeV (LEP II)
Results from precision electroweak measurements:
M(H) = 85 (+39) (-28) GeV
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Higgs @ LHC
Cross-sections 10 to 100 x larger at the LHC (depending on Higgs mass)
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Higgs @ LHC
M(H)<120 GeV Signal x-secion*BR
0.36pbBG x-secion (ttbb)
60pb
Dominant decay channel:H-bb (but only usable in associated production mode)Good b-tagging neededt-tagging to reduce bck
M(H)<140 GeV
2 forward jetsHiggs decay products incentral region
-> 3.4 sigma @ 30 fb-1
-> ~ 40 fb-1 needed for discovery
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Higgs @ LHC
M(H)<150 GeVX-section * BR 50 fb,But very clean signatureEM Calorimetry efficiency Crucial (ATLAS vs CMS performance)
M(H)>130 GeVX-section * BR 5.7 fb,very clean signature
High mass:
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Higgs @ LHC:Discovery potential
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Thank You
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