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Higgs Searches at the Tevatron

Higgs Searches at the Tevatron. Outline. Introduction Tevatron Standard Model (SM) Higgs Introduction Results Low & high mass Combination Non-SM Higgs Introduction Results Prospects & Conclusions. (See parallel sessions for full details). [ Thanks to all my Tevatron colleagues ].

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Higgs Searches at the Tevatron

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  1. Higgs Searches at the Tevatron

  2. Outline • Introduction • Tevatron • Standard Model (SM) Higgs • Introduction • Results • Low & high mass • Combination • Non-SM Higgs • Introduction • Results • Prospects & Conclusions (See parallel sessions for full details) [ Thanks to all my Tevatron colleagues ] Gavin Davies – SUSY08

  3. Tevatron Performance • Over 4fb-1 delivered per experiment. Over 0.2fb-1 recorded last month. • Expect ~6fb-1 by Apr 2009, and 8fb-1 by Oct 2010. Average data-taking efficiency > 85% Results presented here use up to 2.4fb-1 (3fb-1 results at ICHEP) Many thanks to Accelerator Division 05/02 05/04 05/06 05/08 Gavin Davies – SUSY08

  4. Standard Model Higgs • Higgs mechanism • Additional scalar field in SM Lagrangian  mass to W,Z & fermions • Predicts neutral, spin 0 boson • But not its mass • Direct searches at LEP2 • mH> 114.4 GeV • Improved mt & mw tighten indirect constraints: • mH< 160 GeV (EW fit) • mH < 190 GeV if LEP2 limit included  A light (SUSY?) Higgs is favoured Gavin Davies – SUSY08

  5. Excluded SM Higgs Production & Decay • Small production cross-sections • 0.1 -1 pb • Branching ratio dictates search • mH< 135 GeV • gg → H →bb overwhelmed by multijet (QCD) background • Associated WH & ZH production with H →bb decay • Main backgrounds: W+bb, Z+bb, W/Z jj, top, di-boson, QCD • mH> 135 GeV • gg →H → WW • Main background: WW Gavin Davies – SUSY08

  6. SM Higgs Topologies • Low mass: • Intermediate mass: • High mass: Leptonic decay of W / Z boson provides ‘handle’ for event. H  bb reduce SM background High PTopposite sign leptons. Missing ET Gavin Davies – SUSY08

  7. SM Higgs - snapshot • Higgs searches at a hadron machine challenging • Requires • Efficient triggering, lepton ID, b-tagging, jet resolution.. • Sizeable data-sets • Well established • Analyses use multivariate techniques • Neural net (NN), matrix element (ME).. • Regular Tevatron combinations • Approaching the SM expectation • Rapid improvement well beyond √L gain • Even better to come Gavin Davies – SUSY08

  8. Highest associated production cross-section • Use electron and muon channels • Selection • Isolated lepton, missing ET (MET), 2 jets • Backgrounds • W+jets, QCD, top, di-boson • Analyses • Separate channels: 1 & 2 b-tags, e or m, central or forward • NN (and ME) based analyses using kinematic variables Gavin Davies – SUSY08

  9. Exp/SM = 6.4 (14% improvement) • Cross section limits derived from NN distributions • Compare observed (obs) or expected (exp) limit to SM prediction • Recent CDF analysis • Including isolated tracks • Increases acceptance by 25% mH=115GeV Gavin Davies – SUSY08

  10. Large cross-section & acceptance but hard • Selection • Two jets, MET (not aligned in f with jets) • Backgrounds • Physics: W/Z + jets, di-boson, top – from Monte Carlo • Instrumental: Mis-measured MET with QCD jets – from data • Analyses • Optimised b-tagging, NN or decision tree (DT) discriminant Exp/SM~8(both expts.) Gavin Davies – SUSY08

  11. And…. • Not forgetting .. ZH → llbb • 1 and 2 b-tags, NN event selection • Exp/SM ~ 16-20 at 115GeV (1fb-1) • Sensitivity of existing channels rapidly improving • Add additional channels • DØ • Exp/SM ~ 45 at 120GeV • CDF • Exp/SM ~ 40 at 120GeV • Exp/SM ~ 25 at 115GeV • Adds 10% to CDF combination (≡ 20% more data) Gavin Davies – SUSY08

  12. Intermediate mass range • Selection • 2 like-sign high PT leptons (ee, em, mm) • Backgrounds • WW,WZ, charge flips (from data) • Analyses • DØ 1fb-1 and CDF 1.9fb-1 • Exp /SM~25 at 140GeV • Exp /SM ~20 at 160GeV Gavin Davies – SUSY08

  13. Dominant for mH >135GeV • Selection • 2 isolated, opposite sign leptons,MET • Backgrounds • Drell-Yan, W+jets/QCD, tt, SM WW • Analyses: • WW from spin 0 Higgs • Leptons prefer to point in same direction • Use NN, using kinematic variables and ME as inputs Gavin Davies – SUSY08

  14.  • Matrix element • Use observed leptons & missing ET (xobs) • Integrate over LO theory predictions for WW, ZZ, W+g, W+jet, Higgs • Construct LR discriminant from probabilities • Input ME information to NN • CDF – 4 ME’s and 6 kinematic variables • DØ – 1 ME and more kinematic variables • Cross section limit derived from NNoutput distribution LR (H  WW) Gavin Davies – SUSY08

  15. Expected Limit / SM ~ 2.4 Observed Limit / SM ~ 2.1 Expected Limit / SM ~ 2.5 Observed Limit / SM ~ 1.6 Closing in on the SM Gavin Davies – SUSY08

  16. Combination • Compare background and signal plus background hypotheses using Poisson likelihoods • Systematics included in likelihoods • Combine across channels within an experiment • Combine across experiments • Since 2005 sensitivity improved by a factor of 1.7 beyond √L gain Gavin Davies – SUSY08

  17. f f f f b f b f Non-SM Higgs • MSSM • 2 Higgs doublets, ratio of vev’s = tanb • 2 charged (H)and 3 neutral Higgs particles: f =h, H, A • At large tan b • Coupling to d-type fermions enhanced by tanb → sαtan2b • sf  2 sA • For low & intermediate masses • Br(f → bb) ~90%, Br(f → tt) ~10% f → ttand bf →bbborbf →btt topologies Gavin Davies – SUSY08

  18. Neutral MSSM Higgs tt • Smaller BR but cleaner • Signal: (eτhad) (μτhad) (eμ) pairs + MET • Background: Z→tt, jet fakes • Use visible mass: • No significant excess in visible mass, so proceed to set limits • Initially on sxBr then interpret in MSSM (FeynHiggs – arXiv:0705.0746v2) arXiv:0805.2491 Gavin Davies – SUSY08

  19. Neutral MSSM Higgs tt • MSSM interpretation • Width included • Limits very similar for m<0 • Expect to reach tanb~20 at low mA by end 2010 • Most powerful constraints on tanb Gavin Davies – SUSY08

  20. Neutral MSSM Higgs  bb + b[b] • Signal • At least 3 b-tagged jets • Look for peak in dijet mass • Challenge - Large multijet background • Estimate from 2 b-tagged data and simulation • Composition: Sec. vertex mass (CDF) & fit to multiple b-tagging criteria (DØ) • No significant excess, so set limits, initially on sxBr then interpret in MSSM • arXiv:0805.3556 Gavin Davies – SUSY08

  21. Neutral MSSM Higgs  bb + b[b] • MSSM interpretation • Width included Gavin Davies – SUSY08

  22. Going further… • Charged Higgs • Separate t  W+b & t  H+b (H+ cs) via mass templates • And beyond MSSM • Fermiophobic Higgs • Doubly charged Higgs • arXiv:0803.1514 • arXiv:0805.1534 HR > 127 GeV HL > 150 GeV CDF also looked for e/t final states Gavin Davies – SUSY08

  23. Prospects – SM Higgs • Rapid evolution • Significantly better than √L • Increased trigger efficiency, lepton acceptance, improved b-tagging, NN discriminants • For 2010 expect, beyond √L gain • x 1.4 improvement at high mass and x 2.0 improvement at low mass mH =160GeV mH =115GeV Gavin Davies – SUSY08

  24. Prospects – SM Higgs (cont) • End 2009 • Very limited allowed range • End 2010 • Full exclusion • 3 evidence possible over almost entire range Assumes two experiments Gavin Davies – SUSY08

  25. ICHEP08? Conclusions • Tevatron and CDF/ DØexperiments performing very well • Over 4fb-1 delivered, with 8fb-1 expected by end of 2010 • New SM analyses keep coming in • Sensitivity almost improving linearly with luminosity • Well established, common effort across the Collaborations • Closing in on the SM • 2008 - go beyond at 160GeV ? • Have a clear roadmap to reach desired sensitivity • Wide range of BSM searches, ready for discovery • Expect to reach tanb ~20 at low mA Exciting times ahead – will only get better! Gavin Davies – SUSY08

  26. And… Thank you for your attention Please see the parallel sessions for full details Search for the SM Higgs Boson at DØ - Suyong Choi Search for Charged Higgs Bosons at DØ - Yvonne Peters Search for Charged Higgs in Top Quark Decays at CDF - Geumbong Yu Search for BSM Higgs Bosons at DØ - Ingo Torchiani Gavin Davies – SUSY08

  27. Backup slides Gavin Davies – SUSY08

  28. CDF and DØ experiments • Both detectors extensively upgraded for Run IIa • New silicon vertex detector • New tracking system • Upgraded muon chambers • CDF: New plug calorimeter & ToF • DØ • New solenoid & preshowers • Run IIb: New inner tracking layer & L1 trigger Gavin Davies – SUSY08

  29. DØ data taking Gavin Davies – SUSY08

  30. Total H →WW →lvlv WH → lnbb ZH →llbb ZH → llbb Gavin Davies – SUSY08

  31. B-tagging • Critical for low mass H bb • Improves S/B by > 10 • Use lifetime information • Correct for MC / data differences • Measured at given operating points • Analyse separately (“tight”) single & (“loose”) double tags CDF: Secondary vertex reconstruction • Neural Net - improves purity • Inputs: track multiplicity, pT, vertex • decay length, mass, fit • Loose = 50% eff, 1.5 % mistag • Tight = 40% eff, 0.5 % mistag DØ: Neural Net tagger • Secondary vertex & dca based inputs, derived from basic taggers • High efficiency, purity • Loose = 70% eff, 4.5% mistag • Tight = 50% eff, 0.3% mistag Gavin Davies – SUSY08

  32. DØ B-tagging Several mature algorithms used: • 3 main categories: - Soft-lepton tagging - Impact Parameter based - Secondary Vertex reconstruction Gavin Davies – SUSY08

  33. B-tagging – (DØ) Certification • Have MC / data differences – particularly at a hadron machine • Measure performance on data • Tag Rate Function (TRF) Parameterized efficiency & fake-rate as function of pT and η • Use to correct MC b-tagging rate • b and c-efficiencies • Measured using a b-enriched data sample • Fake-rate • Measured using QCD data Gavin Davies – SUSY08

  34. Tau ID • CDF: Isolation based • Require 1 or 3 tracks, pT > 1GeV in the isolation cone • For 3 tracks total charge must be ±1 • pThad > 15 (20) GeV for 1 (3) prongs • Mhad < 1.8 (2.2) GeV • Reject electrons via E/p cut • Validated via W/Z measurements • Performance • Efficiency ~ 40-50% • Jet to tau fake rate ~0.001-0.005 • DØ: 3 NN’s for each tau type • Validated via Z’s Efficiency (NN>0.9): t ~0.65, jets ~0.02 Gavin Davies – SUSY08

  35. Cleanest channel, but low cross section • Selection: • Loose lepton ID, 2 jets • Backgrounds: • Z+jets, top, WZ, ZZ, QCD • Analyses • Constrain dijet mass • NN event selection • Exp/SM ~ 16 Gavin Davies – SUSY08

  36. Limit Setting LEP: Low background, small systematics Tevatron/LHC: High background, large systematics • Systematics, including correlations, taken into account • Main systematics (depending on channel): - Luminosity and normalisation - QCD background estimates - Input background cross-sections - Jet energy scale and b-tagging - Lepton identification - K-factors on W/Z + heavy flavour • Systematic uncertainties included in likelihood via gaussian smearing of expectation (‘profile likelihood’) • Background constrained by maximising profile likelihood (‘sideband fitting’) • Tevatron experiments use LEP CLs (modified frequentist) and Bayesian methods • Limit setting approaches agree to within 10% Gavin Davies – SUSY08

  37. Spring 08 SM Combination Channel Lumi /Technique Final state Total of 28CDF + DØ channels combined Gavin Davies – SUSY08

  38. Spring 08 SM Combination Gavin Davies – SUSY08

  39. Spring 08 SM Combination Gavin Davies – SUSY08

  40. MSSM benchmarks • Five additional parameters due to radiative correction • MSUSY (parameterizes squark, gaugino masses) • Xt (related to the trilinear coupling At→ stop mixing) • M2 (gaugino mass term) •  (Higgs mass parameter) • Mgluino(comes in via loops) • Two common benchmarks • Max-mixing - Higgs boson mass mh close to max possible value for a given tan • No-mixing - vanishing mixing in stop sector → small mass for h Gavin Davies – SUSY08

  41. Neutral MSSM Higgs tt Limits: s x Br (ftt) Evolution: Full 2010 dataset (2 expts.) Width: Gavin Davies – SUSY08

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