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The elusive agent of electroweak symmetry breaking
- an experimentalist point of view -
Ulrich HeintzBrown University
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the holy grail of particle physics
3/11/2008 Ulrich Heintz - Sakurai Prize Celebration
the quest for the Higgs boson atLEP
TevatronLHC
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Gerry’s particle
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H searches before LEP• experiments at PSI, SPS, CESR
• excluded mH < 5 GeV– subject to significant uncertainties
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CUSB, PRD 35, 2883 (1987)X+exclude 2m < mH < 5 GeV
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LEP
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e+e collisions 1989-1995: Ecom = MZ
1996-2000: Ecom 209 GeV
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LEP 2LEP 1
H production at LEP
Center of mass energy (GeV)
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Björken process Higgsstrahlung
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Very little background expected
Events expected at LEP
among 2x107 Z
H search at LEP1
for mH<2mW : H bb (~80%)
0.0 mH 65 GeV/c2
Excluded at 95% C.L.
for mH<2m : H ee,
for mH<2mb: H gg, cc,
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H searches at LEP2• Decay signature defined by Z decay
Higgs Z Fraction bb qq 51.5% bb 14.7% Any ll 6.7% bb 2.5% qq 5.0% Total 80.9%
“4-jets”“missing E” “leptons”
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and then…
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discovery or fluctuation?• 3 selections with increasing purity for mH = 115 GeV
“the results of the experiment were inconclusive so we had to use statistics...”from Louis Lyon’s book “statistics for nuclear and particle physicists”
Data 5Bkg 3.9Sig 3.8
Data 17Bkg 15.8Sig 7.1
Data 4Bkg 1.2Sig 2.2
S/B for mH > 109 GeV/c2
> 0.5 >1 >2
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the final LEP limit
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modified frequentist confidence levels
)( obsb LLPCL
define likelihood ratio
)|()|(
bdatapbsdatapL
)( obsbs LLPCL
for background
signal + background
b
bss CLCLCL
Signal can be excluded at 95% CL if CLs < 0.05
how much less likely is observed data for s+b
than for b only?
mH > 114 GeV
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the end of LEP
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2 km
Tevatron
pp collisions Ecom = 2 TeVL = 41032/cm2/s (2 fb-1/year)analyzed now: Ldt = 5 fb-1
expect Ldt = 10 fb-1 by end of 2011
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H at the Tevatron?
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H production at Tevatron
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gluon fusion439 fb
associate productionWH: 51 fbZH: 33 fb
vector boson fusion39 fb
@MH=160 GeV
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H search at Tevatron
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mH < 135 GeV (Hbb) background drivenWHlbb, ZHll/bb
mH > 135 GeV (HWW) rate drivenall production modes
“low mass” Higgs “high mass” Higgs
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H search at Tevatron• example channel: WHlbb
+ 2 b-tags
e/ + pT + 2-3 jetsfor mH = 115 GeVexpect 24 Higgs decays in 84000 events
expect 7 Higgs decaysin 700 events
use several variables to form neural network discriminant
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H search at Tevatronneural network
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single top production• same final state as low mass Higgs• slightly larger cross section
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Tevatron H limits
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published inPRL 104, 061802 (2010)by CDF and D0 Collaborations
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sensitivity of H search at Tevatron• assume analysis will stay the same• known improvements will be implemented
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146-183
136-190
“low mass” Higgs “high mass” Higgs
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LHC pp collisions end 2011: Ecom = 7 TeV, Ldt = 1 fb-1
end 2014: Ecom = 14 TeV, Ldt = 25 fb-1
end 2016: Ecom = 14 TeV, Ldt = 100 fb-1
Peter Jenni (Moriond 2010)
CMS
Alice
ATLAS
LHCb
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H production at LHC Higgs production cross sections at LHC
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CMS projections for H search• mH < 130 GeV
– H
• 130 < mH < 190 GeV– H W+W- ℓℓ
• 190 < mH < 460 GeV– H ZZ ℓℓ ℓℓ
• 460 < mH < 800 GeV– H W+W- ℓqq
designed to discover Higgs with 100 < mH < 800 GeV3/11/2008 Ulrich Heintz - Sakurai Prize Celebration
hard to see: H bb
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the CMS experiment• homogeneous electromagnetic calorimeter
– 78,000 lead tungstate crystals– excellent photon energy resolution– measure H
M (GeV)
mH = 130 GeVL = 100 fb-1
# of
eve
nts
/ 0.5
GeV
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LHC projection for next 2 years
exclude 145 < mH < 190
GeV
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LHC projection for next 2 years
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summary
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• the search for Gerry’s boson has shaped particle physics experiments for the last three decades
• but it still eludes detection• the quest goes on…
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Congratulations, Gerry!
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