1. Higgs Reach Through VBF with ATLAS Bruce Mellado University of Wisconsin-Madison Recontres de Moriond 2004 QCD and High Energy Hadronic Interactions

2. Bruce Mellado, Moriond 2004, 02/04/042 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. Bruce Mellado, Moriond 2004, 02/04/043 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 VBF gg  H Associated

4. Bruce Mellado, Moriond 2004, 02/04/044 Low Mass Higgs via VBF Wisconsin Phenomenology Institute (D.Rainwater, D.Zeppenfeld et al.):  Two high P T jets with large  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 Jet   Forward jets Higgs Decay

5. Bruce Mellado, Moriond 2004, 02/04/045 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 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

6. Bruce Mellado, Moriond 2004, 02/04/046 Low Mass Higgs via VBF H->WW*->ll,l qq. Strongest in 125<M H <190 GeV  Main background:  tt EW WWjj  W + 4 jets H->  ->ll,lh (+ptmiss). Strong around LEP limit  Main background  QCD and EW Zjj H-> . Contributes around LEP limit  Main background  Real and fake non-resonant 

7. Bruce Mellado, Moriond 2004, 02/04/047 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 veto  Lepton angular correlations Two neutrinos in final state. No narrow mass peak, transverse mass is used, instead  May achieve large signal-to-background ratio ATLAS Preliminary Used MC@NLO for tt production

8. Bruce Mellado, Moriond 2004, 02/04/048 Perturbative QCD Parton Shower H->WW*->ll (cont) ATLAS updated study with new simulation of tt  Old study based on Pythia  New study based on MC@NLO/HERWIG Strong differences between generators  P T of tt and jet multiplicity  However, discovery potential does not suffer significantly Made mass dependent cut optimization

9. Bruce Mellado, Moriond 2004, 02/04/049 H->WW*->ll (cont) Evidence of Spin-0 resonance in H->WW->ll modes  Look into difference in  between leptons M T >175 GeVM T <175 GeV Signal RegionOutside Signal Region

10. Bruce Mellado, Moriond 2004, 02/04/0410 H  analyses (  ->ll,lh):  M  reconstruction using collinear approximation (mass resolution ~10%)  Missing E T and  reconstruction, critical issues  Background ( Zjj) rejection  Well separated forward jets and central jet veto H->  Collinear approximation H->  ->ll 30 fb -1 ATLAS CMS

11. Bruce Mellado, Moriond 2004, 02/04/0411 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 H->  ATLAS Real Fake

12. Bruce Mellado, Moriond 2004, 02/04/0412 Intermediate Mass Higgs via VBF Used for first time H  WW  ll and H  ZZ  llqq associated with two hard jets for intermediate masses (2M Z <M H <500 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. Bruce Mellado, Moriond 2004, 02/04/0413 Sensitivity to low mass SM Higgs dominated by VBF VBF studies extended to intermediate masses ATLAS Preliminary

14. Bruce Mellado, Moriond 2004, 02/04/0414 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->  ->ll  Similar results  Results improve by 45-50%  5  effect for M H >115 GeV with one exp and 10 fb -1 provided nominal detector performance H->WW->ll ATLAS

15. Bruce Mellado, Moriond 2004, 02/04/0415 MSSM Higgs Discovery Potential Two Experiments 10 fb -1 (No VBF) One Experiment 10 fb -1 (With VBF)

16. Bruce Mellado, Moriond 2004, 02/04/0416 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. Bruce Mellado, Moriond 2004, 02/04/0417 Couplings Measurement (cont) Relative Branching Ratios Relative Couplings

18. Bruce Mellado, Moriond 2004, 02/04/0418 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 M H >115 GeV with one experiment and 10 fb -1 assuming expected detector performance  ATLAS has extended these searches to 2M Z <M H <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