1 Physics at Hadron Colliders Lecture III Beate Heinemann University of California, Berkeley and Lawrence Berkeley National Laboratory CERN, Summer Student Lectures, 2008
Jan 08, 2016
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Physics at Hadron Colliders
Lecture III
Beate Heinemann
University of California, Berkeley and
Lawrence Berkeley National Laboratory
CERN, Summer Student Lectures, 2008
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Outline Lecture I: Introduction
Outstanding problems in particle physics and the role of hadron colliders
Current and near future colliders: Tevatron and LHC Hadron-hadron collisions
Lecture II: Standard Model Measurements Tests of QCD Precision measurements in electroweak sector
Lecture III: Searches for the Higgs Boson Standard Model Higgs Boson Higgs Bosons beyond the Standard Model
Lecture IV: Searches for New Physics Supersymmetry High Mass Resonances (Extra Dimensions etc.)
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The Higgs Boson Symmetry breaking caused by
scalar Higgs field vacuum expectation value of the
Higgs field <> =246 GeV/c2 gives mass to the W and Z gauge
bosons, MW gW<> fermions gain a mass by Yukawa
interactions with the Higgs field, mf gf<>
Higgs boson couplings are proportional to mass
Higgs boson prevents unitarity violation of WW cross section (ppWW) > (pp anything)
=> illegal! Something new must happen at LHC
Peter Higgs
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Higgs Production: Tevatron and LHC
dominant: gg H, subdominant: HW, HZ, Hqq
LHCTevatron
(pb
)
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Higgs Boson Decay
Depends on Mass MH<130 GeV/c2:
bb dominant WW and subdominant small but useful
MH>130 GeV/c2: WW dominant ZZ cleanest
BR
bb
WWZZ
LEP excluded
_
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Describe what significance means…
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High Mass: mH>140 GeV
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Higgs mass reconstruction impossible due to two neutrinos in final state
Make use of spin correlations to suppress WW background:
Higgs has spin=0leptons in H WW(*) l+l- are collinear
Main background: WW production
H WW(*) l+l- _
10x 160 GeV Higgs
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Event selection: 2 isolated e/:
pT > 15, 10 GeV Missing ET >20 GeV Veto on
Z resonance Energetic jets
Separate signal from background Use matrix-element or Neural
Network discriminant to Main backgrounds
SM WW production Top Drell-Yan Fake leptons
New result!
e
HWW(*)l+l- (l=e,
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High Mass Higgs Signals
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CMS HZZ eeee example signals for mH=150 GeV/c2
HZZ*
HWW*
ATLAS
M. Dührssen et al., hep-ph/0504006
Clean signals on rather well understood backgrounds
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Low Mass: mH<140 GeV
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WH selection: 1 or 2 tagged b-jets electron or muon
with pT > 20 GeV ET
miss > 20 GeVe/
b jet
b jet
Now looking for 2 jets
Expected Numbers of Events: WH signal: 0.85 + 0.65Background: 62±13 + 69±12
WHlbb
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ZHbb
Event selection: ≥ 1 tagged b-jets Two jets ET
miss > 70 GeV
Lepton veto Veto missing ET along jet
directions
Big challenge: Background from mismeasurement
of missing ET
QCD dijet background is HUGE Generate MC and compare to data
in control regions Estimate from data
Control: Missing ET direction Missing ET in hard jets vs overall
missing ET
jet
jet
jet
jetET
ET
mismeasured genuine
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QCD Jet Background to ZHbb DØ uses data
Define variable that can be used to normalize background
Asymmetry between missing ET inside jets and overall missing ET
Sensitive to missing ET outside jets
Background has large asymmetry Signal peaks at 0
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Background understanding using MC
CDF use MC and check it in detail against data
“QCD” control region:Jet aligned with missing ET
Completely dominated by QCD jets and mistags
“EWK” control region:Identified lepton in event=> Dominated by top
Look at data only when control regions look satisfactory
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Dijet Mass distributions
Backgrounds still much larger than the signal: Further experimental improvements and luminosity required E.g. b-tagging efficiency (40->60%), NN selection, higher lepton
acceptance
H signal x10 H signal
WHlbb
ZHbbZHllbb
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Higgs Search with Neural Network
Construct neural network can be powerful to improve discrimination: Here 10 variables are used in 2D
Neural Network
Critical: understanding of distribution in control
samples
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no b-tags
2 b-tags
1 b-tag
SM(ZH)x19
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LHC: Low Mass Region
LHC for mH=115 GeV/c2
L≈10 fb-1 for 5 discovery for single experiment CMS mostly sensitive to decay ATLAS more sensitive to ttH->ttbb and qqH->qq
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115 GeV Higgs: LHC with 10fb-1
H-> ttH->ttbb qqH->qq
S 150 15 ~10
B 3900 45 ~10
S/√B 2.4 2.2 ~2.7
Large K-factor~2 not included
L=30fb-1
Total S/√B=4.2 First evidence possible
Difficult to know whether it is the Higgs boson Important to see signal in each channel
Gives first idea about branching ratios Diphoton channel will have nice peak, others not
ATLAS
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130GeV Higgs: first year (10fb-1)
complete detector
H-> H->ZZ qqH->qqWW
qqH->qq
S 120 5 18 ~8
B 2500 <1 15 ~6
S/√B 2.4 2.8 3.9 2.6
Total S/√B=6 This is good!
Now qqWW and ZZ channels contribute a lot ttH channel really difficult now => cannot measure branching into b’s Nice peaks expected in and ZZ
130 GeV HiggsL = 100 fb-1
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Low Mass Higgs Signals
ATLAS, 100 fb-1
H
VBF H CMS, 30 fb-1
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H challenges:Background from 0Mass resolution requires brilliant calibrationAt least 1 photon converts in 50% of events
VBF: Hqqqq challenges:Central jet veto sensitive to underlying eventNeed to understand forward jetsBackground from jets looking like tau’s
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Ratio to Standard Model
Further experimental improvements and luminosity expected Will help to close the gap Expect to exclude 160 GeV Higgs boson soon At low mass still rather far away from probing SM cross section
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LHC SM Higgs Discovery Potential
2004
Fast discovery for high mass, e.g. mH>150 GeV/c2
Harder at low mass=> zoom into low mass region
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Ultimate sensitivity
With 6 fb-1 of LHC data will know if Higgs boson exists in 2-3 years already (hopefully)!
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∫ L dt= 6 fb-1
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How do we know what we have found?
After discovery we need to measure: The mass The spin The branching ratio into all fermions
Verify coupling to mass
The total width Are there invisible decays?
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Mass
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Couplings at LHCDuehrssen et al hep-ph/0407190
Measure the couplings of the Higgs to as many particles as possible:
H->ZZH->H->WWH->H->bb
And in different production modes:gg->H, ttH (tH coupling)WW->H (WH coupling)
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Non Standard-Model Higgs Bosons
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Minimal Supersymmetric Standard Model: 2 Higgs-Fields: Parameter tan=<Hu>/<Hd> 5 Higgs bosons: h, H, A, H±
Neutral Higgs Boson: Pseudoscalar A Scalar H, h
Lightest Higgs (h) very similar to SM
Higgs in the MSSM
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MSSM Higgs Selection
pp +X +X : One decays to e or One decays to hadrons or e/ Use “visible mass”, m(ET,l1,l2) for
discrimination against background
pp b+X bbb+X : Three b-tagged jets
ET>35, 20 and 15 GeV
Use invariant mass of leading two jets to discriminate against background
=h/H/A
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Mass Distributions
Good agreement between data and background in all analyses
No sign of deviation
e+ e/ e/
bbb
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MSSM Higgs: Results
pp A+X +X Sensitivity at high tan Exploting regime beyond LEP
pp Ab+X bbb+X Probes high tan if<0 Combined with channel by D0
Future (L=8 fb-1): Probe values down to 25-30!
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at least one Higgs boson
observable for all parameters
significant area where only lightest Higgs boson h is observable
can SM be discriminated from
extended Higgs sector by
parameter determination?
300 fb-1
MSSM Higgs Bosons at LHC
At least one Higgs boson observable in all modelsOften only one Higgs Boson observableCould also be produced in SUSY cascades:
Depends on model how well this can be exploited
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Conclusions The Higgs boson is the last missing piece in
the Standard Model And arguably the most important SM particle
Searches ongoing at the Tevatron Chance of a 3sigma evidence
LHC will find the Higgs boson if it exists And measure some of it’s properties
There might be more than one Higgs boson E.g. in supersymmetry They can be found too
35dominant: gg H, subdominant: HW, HZ
(pb
)
e/
b jet
b jet
Higgs Production at the Tevatron