Higgs Reach Through VBF with ATLAS

Bruce MelladoUniversity of Wisconsin-Madison

Recontres de Moriond 2004QCD and High Energy Hadronic

Interactions

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OutlineIntroductionHiggs Discovery PotentialSM Higgs

Low Mass SM HiggsIntermediate Mass SM HiggsOverall ATLAS potential

MSSM Higgs

Prospects of Couplings MeasurementConclusions

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Production:Direct

gg H Dominant Large background

at masses close to LEP limit

qq qqH (VBF) Distinct final state

Associated ttH, WH, ZH

Small cross-section

SM Higgs at LHC

VBFgg H

Associated

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Low Mass Higgs via VBFWisconsin Phenomenology Institute (D.Rainwater, D.Zeppenfeld et al.):Two high PT jets with

large separationStrong discovery

potential for low Higgs mass

Helps measuring couplings

Feasibility studies CMS Note 2003/033 ATLAS SN-ATLAS-2003-024

Updates in progress

Jet

Jet

Forward jets

Higgs Decay

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Tagging forward jets:Efficiencies criticalFull simulation usedDouble tag efficiency ~50%

Central jet veto:Pile up effects introduce fake central jets Effect small at low

luminosity Serious concern at high

luminosity

Major Experimental Issues

Major experimental issues addressed with a full detector simulation (Geant3)

Currently ATLAS is assessing feasibility of entire analysis with a full detector simulation

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Low Mass Higgs via VBF

H->WW*->ll,lqq. Strongest in 125<MH<190 GeVMain background:

tt EW WWjj W + 4 jets

H->->ll,lh (+ptmiss). Strong around LEP limitMain background

QCD and EW Zjj

H->. Contributes around LEP limitMain background

Real and fake non-resonant

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H->WW*->ll

Background: tt, EW WWjj Understanding of tt production is crucial

Background suppression:Well separated forward jets and central jet veto

b-jet vetoLepton angular correlations

Two neutrinos in final state. No narrow mass peak, transverse mass is used, insteadMay achieve large signal-to-background ratio

ATLAS Preliminary

Used MC@NLO for tt production

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Perturbative QCD

Parton Shower

H->WW*->ll (cont)

ATLAS updated study with new simulation of ttOld study based on PythiaNew study based on MC@NLO/HERWIG

Strong differences between generatorsPT of tt and jet multiplicity

However, discovery potential does not suffer significantly

Made mass dependent cut optimization

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H->WW*->ll (cont)

Evidence of Spin-0 resonance in H->WW->ll modesLook into difference in between leptons

MT>175 GeVMT<175 GeV

Signal Region

Outside Signal Region

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H analyses (->ll,lh):M reconstruction using collinear approximation (mass resolution ~10%) Missing ET and reconstruction, critical

issues Background (Zjj) rejection

Well separated forward jets and central jet veto

H-> Collinear approximation

H-> ->ll30 fb-1

ATLAS

CMS

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Narrow peak with ~1.4% mass resolutionBackgroundsReal and fake photons

Suppression Well separated forward

jets and central jet veto

Diminished by large fake photon contribution~2 for 30 fb-1

Needs more work

H->

ATLAS

Real

Fake

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Intermediate Mass Higgs via VBF

Used for first time HWWll and HZZllqq associated with two hard jets for intermediate masses (2MZ<MH<500 GeV)

By using kinematic fits obtain (M/M2.5%) with HZZllqq

Discovery confirmation and direct measurement of couplings ratio

ATLAS Preliminary

HZZ

HWW

ΓΓ

=)qqZZqqH(BR×σ)qqWWqqH(BR×σ

→

→

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Sensitivity to low mass SM Higgs dominated by VBFVBF studies extended to intermediate masses

ATLAS Preliminary

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Multivariate Analysis

Signal significance improvement with neural network based analysis: Neural network output

used as a discriminating variable with likelihood techniques

NN applied to H->WW->ll and H->->llSimilar results

Results improve by 45-50%

5 effect for MH>115 GeV with one exp and 10 fb-1 provided nominal detector performance

H->WW->ll

ATLAS

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MSSM Higgs Discovery Potential

Two Experiments 10 fb-1 (No VBF)

One Experiment 10 fb-1 (With VBF)

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Couplings MeasurementATLAS attempted a global fit to cross-sections of known channels to extract Higgs couplingsHiggs searches associated with two hard jets play a big role

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Couplings Measurement (cont)

Relative Branching Ratios

Relative Couplings

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SummarySearches associated with two hard jets dominate sensitivity for low mass HiggsForward jet tagging efficiency crucial and understanding of central jet veto - crucial issues

With Neural nets and likelihood techniques may reach 5 effect for MH>115 GeV with one experiment and 10 fb-1 assuming expected detector performance

ATLAS has extended these searches to 2MZ<MH<500 GeV

With these searches one experiment may cover all MSSM parameter space with 10 fb-1

Efforts underway to address entire analysis with a full detector simulation wit systematic error studies with “data-like” control samples

SM Higgs Coupling measurements (low mass):Accuracy of relative branching ratios and relative couplings vary from 10% to 60% depending on coupling and mass