Physics at Hadron Colliders Lecture III

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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

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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





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



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)!



∫ 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

Physics at Hadron Colliders Lecture III

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