Higgs Reach Through VBF with ATLAS

1
Higgs Reach Through VBF with ATLAS

• Bruce Mellado
• University of Wisconsin-Madison

Recontres de Moriond 2004 QCD and High Energy
Hadronic Interactions
2
Outline

• Introduction
• Higgs Discovery Potential
• SM Higgs
• Low Mass SM Higgs
• Intermediate Mass SM Higgs
• Overall ATLAS potential
• MSSM Higgs
• Prospects of Couplings Measurement
• Conclusions

3
SM Higgs at LHC

• 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

4
Low Mass Higgs via VBF

• Wisconsin Phenomenology Institute (D.Rainwater,
  D.Zeppenfeld et al.)
• Two high PT jets with large Dh separation
• Strong discovery potential for low Higgs mass
• Helps measuring couplings
• Feasibility studies
• CMS Note 2003/033
• ATLAS SN-ATLAS-2003-024
• Updates in progress

5
Major Experimental Issues

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

• Tagging forward jets
• Efficiencies critical
• Full simulation used
• Double tag efficiency 50
• Central jet veto
• Pile up effects introduce fake central jets
• Effect small at low luminosity
• Serious concern at high luminosity

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

6
Low Mass Higgs via VBF

• H-gtWW-gtll??,l?qq. Strongest in 125ltMHlt190 GeV
• Main background
• tt EW WWjj
• W 4 jets
• H-gt??-gtll,lh (ptmiss). Strong around LEP limit
• Main background
• QCD and EW Zjj
• H-gt??. Contributes around LEP limit
• Main background
• Real and fake non-resonant ??

7
H-gtWW-gtll??

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

ATLAS Preliminary

• Background tt, EW WWjj
• Understanding of tt production is crucial
• Background suppression
• Well separated forward jets and central jet veto
• b-jet veto
• Lepton angular correlations

Used MC_at_NLO for tt production
8
H-gtWW-gtll?? (cont)

• ATLAS updated study with new simulation of tt
• Old study based on Pythia
• New study based on MC_at_NLO/HERWIG

• Strong differences between generators
• PT of tt and jet multiplicity
• However, discovery potential does not suffer
  significantly
• Made mass dependent cut optimization

9
H-gtWW-gtll?? (cont)

• Evidence of Spin-0 resonance in H-gtWW-gtll modes
• Look into difference in ? between leptons

Signal Region
Outside Signal Region
MTgt175 GeV
MTlt175 GeV
10
H-gt??
Collinear approximation

• H??? analyses (??-gtll,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-gt ??-gtll 30 fb-1
CMS
ATLAS
11
H-gt??

• Narrow peak with 1.4 mass resolution
• Backgrounds
• Real 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

ATLAS
12
Intermediate Mass Higgs via VBF

• Used for first time H?WW?ll and H?ZZ?llqq
  associated with two hard jets for intermediate
  masses (2MZltMHlt500 GeV)
• By using kinematic fits obtain (?M/M?2.5) with
  H?ZZ?llqq
• Discovery confirmation and direct measurement of
  couplings ratio

ATLAS Preliminary
13

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

ATLAS Preliminary
14
Multivariate Analysis
ATLAS

• Signal significance improvement with neural
  network based analysis
• Neural network output used as a discriminating
  variable with likelihood techniques
• NN applied to H-gtWW-gtll and H-gt??-gtll
• Similar results
• Results improve by 45-50
• 5? effect for MHgt115 GeV with one exp and 10 fb-1
  provided nominal detector performance

15
MSSM Higgs Discovery Potential
One Experiment 10 fb-1 (With VBF)
Two Experiments 10 fb-1 (No VBF)
16
Couplings Measurement

• ATLAS attempted a global fit to cross-sections of
  known channels to extract Higgs couplings
• Higgs searches associated with two hard jets play
  a big role

17
Couplings Measurement (cont)
Relative Couplings
Relative Branching Ratios
18
Summary

• Searches associated with two hard jets dominate
  sensitivity for low mass Higgs
• Forward jet tagging efficiency crucial and
  understanding of central jet veto - crucial
  issues
• With Neural nets and likelihood techniques may
  reach 5? effect for MHgt115 GeV with one
  experiment and 10 fb-1 assuming expected detector
  performance
• ATLAS has extended these searches to 2MZltMHlt500
  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