1. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 1 Discovery Potential for the Standard Model Higgs at ATLAS La Thuile, 1-7 March, 2009 Representing the ATLAS Collaboration Glen Cowan RHUL Les Rencontres de Physique de la Vallée d'Aoste

2. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 2 Outline Introduction Standard Model Higgs @ LHC/ATLAS Standard Model Higgs channels considered here: H →   H → WW ( * )  → e   H → ZZ ( * ) → 4l (l = e,  ) H →     → ll, lh Statistical combination of channels Results Conclusions and outlook

3. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 3 Emphasis of this talk Status of Higgs search recently published in: Expected Performance of the ATLAS Experiment: Detector, Trigger and Physics, arXiv:0901.0512, CERN-OPEN-2008-20. Updated since previous studies: improved detector description and MC generators, higher order QCD and EW corrections included. Statistical procedure for combination of channels. Systematics included via profile likelihood In this talk, concentrate on "discovery modes": H →  H → WW ( * )  → e  H → ZZ ( * ) → 4l (l = e,  ) H →     → ll, lh Some modes not yet included, e.g., WW → e e, , l qq; → sensitivity will improve

4. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 4 The Large Hadron Collider CM energy14 TeV (design) LuminosityLow: 2  10 33 (cm  s  )High: 10 34 Bunch crossing24.95 ns Particles/bunch1.15  10 11 Beam radius16.7  m Overlaid events23 @ 10 34 cm  s  Dipole field8.33 Tesla Stored energy360 MJ/beam

5. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 5 The ATLAS Detector HCAL: Fe+scint. / Cu+LAr (10λ), σ(E)/E ≈ 50%/√E + 0.03 GeV Muon: σ(p T )/p T ≈ 2% @ 50GeV to 10% @ 1TeV (ID+MS) Trigger: L1 + RoI-based HLT (L2+EF) Magnet: 2 T solenoid + toroids (0.5 T bar- rel, 1 T endcap) Tracker: Si pixels, strips + TRT, σ(p T )/p T ≈ 5x10 -4 p T + 0.01 ECAL: Pb+LAr, σ(E)/E ≈ 10%/√E + 0.007 G. Aad et al., (The ATLAS Collaboration) J. Instrum. 3 (2008) S08003

6. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 6 Higgs production at the LHC

7. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 7 Vector Boson Fusion VBF (qq → qqH) gives two forward jets, no colour connection → large rapidity gap Additional kinematic properties give background suppression studied in ATLAS for: H → WW, , 

8. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 8 Higgs decay modes bb very difficult WW dominates for 2M W < m H < 2M Z ZZ "golden mode"  important at low mass

9. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 9 H → ZZ ( * ) → 4l (l = e,  ) Effective in a broad mass range for m H > 130 GeV (except small gap between 2M W and 2M Z ). Lepton efficiency crucial:  4l ~  l 4 Require at least one Z on shell. Main triggers: p T (e) > 22 GeV or p T (  ) > 20 GeV Main background: ZZ ( * ) also (reducible) Zbb, tt, suppress with isolation, impact parameter, vertex constraints Fit remaining background from sidebands of m 4l.

10. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 10 H → ZZ ( * ) → 4l sensitivity Expected 95% CL upper limit on  =  /  SM for 5 fb  if no Higgs. Expected discovery significance (# of  ) of SM Higgs for 30 fb  Lower points include systematics median ±1±1 ±2±2

11. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 11 H →     with 2 jets Exploit VBF kinematics to suppress Z+jets, tt back- grounds (Central Jet Veto). Use  → ll, lh (also study  → hh). Higgs mass reconstructed using missing E T and by assuming tau decay products collinear with tau. Main triggers: p T (e) > 22 GeV or p T (  ) > 20 GeV Important backgrounds: Z + jets, tt Z + jets background control: select Z → , replace  by (MC) 

12. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 12 H →     with 2 jets Simultaneous fit of signal and QCD, Z+jets control samples:  → lh gives most sensitivity:

13. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 13 For 115 < m H < 140 GeV, BR ~ 0.002, but distinct signature. Inclusive analysis based on m  Exclusive analyses (H + 0,1,2 jets) also use other variables (event topology, p T, , cos   *); combined in simultaneous fit. Main triggers: 2 photons, each p T > 20 GeV Main backrounds: ,  +jet, jet+jet 57% of H→  events have at least one photon convert (|  |<2.5) Conversion reconstruction efficiency 66.4% (r < 40 cm)  vertex from ECAL pointing and tracker. H → 

14. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 14 H →  Expected discovery significance for inclusive and combination of exclusive channels (H + 0,1,2 jets), here both fixed and floating m H. Inclusive analysis used in current combination with other channels. m  resolution crucial. Note number of entries here for 1 fb  ~ 250 → 6% stat. error per bin.

15. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 15 Most important for 2M W < m H < 2M Z Two cases: no jets (mainly gg fusion) and 2 jets (mainly VBF) For now only WW → e  (e e,  l qq under study) No mass peak because of neutrinos → use transverse mass: H → WW ( * ) where

16. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 16 Simultaneous fit of: transverse mass, p T of WW,  ll (angle between ll in transverse plane) H → WW with 0 jets Correlated W spins cause leptons to go preferentially in same direction: Main triggers: p T (e) > 22 GeV or p T (  ) > 20 GeV Main background: QCD WW  ll

17. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 17 H → WW with 2 jets Main triggers: p T (e) > 22 GeV or p T (  ) > 20 GeV Main background: tt Select signal region in  ll,  ll Simultaneous fit of transverse mass and neural net output based on jet activity

18. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 18 Statistical combination of channels Treat systematics by means of profile likelihood method. Considers fixed-mass hypothesis; "look-elsewhere-effect" must be studied separately. Studies for individual channels also use floating-mass fits. Approximate methods for discovery/exclusion significance. Valid for L > 2 fb . For lower L need MC methods. Does not represent final word on methods -- other developments ongoing (Bayesian, CL s, look-elsewhere-effect,...) In following describe method for binned distributions; some channels also use unbinned fits.

19. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 19 Statistical model Bin i of a given channel has n i events, expectation value is Expected signal and background are:  is global strength parameter, common to all channels.  = 0 means background only,  = 1 is SM hypothesis. b tot,  s,  b are nuisance parameters

20. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 20 The likelihood function  represents all nuisance parameters, e.g., background rate, shapes here signal rate is only parameter of interest The single-channel likelihood function uses Poisson model for events in signal and control histograms: The full likelihood function is There is a likelihood L i ( ,  i ) for each channel, i = 1, …, N. data in signal histogram data in control histogram

21. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 21 Profile likelihood ratio To test hypothesized value of , construct profile likelihood ratio: Maximized L for given  Maximized L Equivalently use q  =  2 ln (  ): data agree well with hypothesized  → q  small data disagree with hypothesized  → q  large Distribution of q  under assumption of  related to chi-square (Wilks' theorem, approximation valid for roughly L > 2 fb  ):

22. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 22 p-value / significance of hypothesized  Test hypothesized  by giving p-value, probability to see data with ≤ compatibility with  compared to data observed: Equivalently use significance, Z, defined as equivalent number of sigmas for a Gaussian fluctuation in one direction:

23. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 23 Sensitivity Discovery: Generate data under s+b (  = 1) hypothesis; Test hypothesis  = 0 → p-value → Z. Exclusion: Generate data under background-only (  = 0) hypothesis; Test hypothesis . If  = 1 has p-value < 0.05 exclude m H at 95% CL. Estimate median significance by setting data equal to expectation values or by using MC. For median, can combine significances of individual channels. For significance of e.g. a real data set, need global fit.

24. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 24 Combined discovery significance

25. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 25 Combined discovery significance Approximations in combination not always accurate for L < 2 fb  but in most cases conservative. Discovery signficance (in colour) vs. L, m H :

26. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 26 Combined 95% CL exclusion limits For 2 fb , expected 95% CL upper limit on m H is 115 GeV.

27. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 27 Combined 95% CL exclusion limits Approximations in combination not always accurate for L < 2 fb  but in most cases conservative.  p-value of m H (in colour) vs. L, m H :

28. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 28 Summary and conclusions Studies here only cover Standard Model Higgs channels most relevant to discovery some modes not yet included → sensitivity will improve Studies use NLO cross sections, improved MC generators, detailed detector simulation, improved statistical treatment. Systematic uncertainties treated using profile likelihood. Due to approximations used, combined results valid for L > 2 fb  For L = 2 fb  : expected discovery 5  or more in 143 < m H < 179 GeV, expected upper limit on m H at 95% CL is 115 GeV

29. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 29 Special thanks to my ATLAS colleagues, especially: Eilam Gross, Aleandro Nisati, Ketevi Assamagan, Bill Quayle, Yaquan Fang, Ricardo Goncalo, Tom LeCompte, Bruce Mellado, Marumi Kado,...

30. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 30 Extra slides

31. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 31 The "look-elsewhere effect" Look for Higgs at many m H values -- probability of seeing a large fluctuation for some m H increased. Combined significance shown here relates to fixed m H. False discovery prob enhanced by ~ mass region explored /  m For H→  and H→WW, studied by allowing m H to float in fit:  → 

32. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 32 Expected discovery significance

33. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 33 Expected p-value of SM vs m H

34. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 34 H → 4l at different m H

35. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 35 H →  for exclusive channels H + 1 jet H + 2 jets H + E T,miss + lepton H + E T,miss

36. G. Cowan / RHUL Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009 36 Example from validation exercise: ZZ (*) → 4l 5  level (One minus) cumulative distributions. Band gives 68% CL limits. ½2½2 ½2½2 ½2½2 ½2½2 Distributions of q 0 for 2, 10 fb  from MC compared to ½  2