1 Higgs Mechanism 13.9.2007 - Cyril Topfel. 2 What to expect from this Presentation (Table of...

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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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