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Discovery Potential for the Standard Model Higgs at ATLAS

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

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Discovery Potential for the Standard Model Higgs at ATLAS

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  1. Discovery Potential for the Standard Model Higgs at ATLAS Les Rencontres de Physique de la Vallée d'Aoste La Thuile, 1-7 March, 2009 Glen Cowan RHUL Representing the ATLAS Collaboration 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 →gg H → WW (*)→ enmn H → ZZ(*) → 4l (l = e, m) H → t+t- → ll, lh Statistical combination of channels Results Conclusions and outlook 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 →gg H → WW (*)→ enmn H → ZZ(*) → 4l (l = e, m) H → t+t- → ll, lh Some modes not yet included, e.g., WW → enen, mnmn, lnqq; →sensitivity will improve Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

  4. The Large Hadron Collider CM energy 14 TeV (design) Luminosity Low: 2  1033 (cm-2s-1) High: 1034 Bunch crossing 24.95 ns Particles/bunch 1.15  1011 Beam radius 16.7 mm Overlaid events 23 @ 1034 cm-2s-1 Dipole field 8.33 Tesla Stored energy 360 MJ/beam Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

  5. The ATLAS Detector G. Aad et al., (The ATLAS Collaboration) J. Instrum. 3 (2008) S08003 Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

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

  8. WW dominates for 2MW < mH < 2MZ Higgs decay modes ZZ "golden mode" bb very difficult tt, gg important at low mass Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

  9. H → ZZ(*)→ 4l (l = e,m) Effective in a broad mass range for mH > 130 GeV (except small gap between 2MW and 2MZ). Lepton efficiency crucial: e4l ~ el4 Require at least one Z on shell. Main triggers: pT(e) > 22 GeV or pT(m) > 20 GeV Main background: ZZ(*) also (reducible) Zbb, tt, suppress with isolation, impact parameter, vertex constraints Fit remaining background from sidebands of m4l. 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 m = s / sSM for 5 fb-1 if no Higgs. median ±1s ±2s Expected discovery significance (# of s) of SM Higgs for 30 fb-1 Lower points include systematics Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

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

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

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

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

  15. H → WW(*) Most important for 2MW < mH < 2MZ Two cases: no jets (mainly gg fusion) and 2 jets (mainly VBF) For now only WW → enmn (enen, mnmnlnqq under study) No mass peak because of neutrinos → use transverse mass: where Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

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

  17. H → WW with 2 jets Main triggers: pT(e) > 22 GeV or pT(m) > 20 GeV Main background: tt Select signal region in Dhll, Dfll Simultaneous fit of transverse mass and neural net output based on jet activity 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-1. For lower L need MC methods. Does not represent final word on methods -- other developments ongoing (Bayesian, CLs, look-elsewhere-effect,...) In following describe method for binned distributions; some channels also use unbinned fits. Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

  19. Statistical model Bin i of a given channel has ni events, expectation value is m is global strength parameter, common to all channels. m = 0 means background only, m = 1 is SM hypothesis. Expected signal and background are: btot, qs, qb are nuisance parameters Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

  20. The likelihood function The single-channel likelihood function uses Poisson model for events in signal and control histograms: data in control histogram data in signal histogram  represents all nuisance parameters, e.g., background rate, shapes here signal rate is only parameter of interest There is a likelihood Li(,i) for each channel, i = 1, …, N. The full likelihood function is 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 qm under assumption of m related to chi-square (Wilks' theorem, approximation valid for roughly L > 2 fb-1): Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

  22. p-value / significance of hypothesized m 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: 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 mH 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. Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

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

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

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

  27. Combined 95% CL exclusion limits 1 -p-value of mH (in colour) vs. L, mH: Approximations in combination not always accurate for L < 2 fb-1 but in most cases conservative. 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-1 For L = 2 fb-1: expected discovery 5s or more in 143 < mH < 179 GeV, expected upper limit on mH at 95% CL is 115 GeV 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,... Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

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

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

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

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

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

  35. H →gg for exclusive channels H + 1 jet H + 2 jets H + ET,miss H + ET,miss + lepton Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

  36. Example from validation exercise: ZZ(*)→ 4l Distributions of q0 for 2, 10 fb from MC compared to ½2 ½2 ½2 (One minus) cumulative distributions. Band gives 68% CL limits. ½2 ½2 5 level Discovery Potential for the SM Higgs at ATLAS / La Thuile, 1-7 March 2009

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