html5-img
1 / 39

On behalf of the

Higgs Searches at Tevatron. M axim Titov, CEA Saclay , France. On behalf of the. COLLABORATIONS. 18 August 2011, Moscow, Russia. Reaching the Higgs Horizon. Introduction Challenges and Analysis strategies Standard Model Higgs Searches New Tevatron combined result

zyta
Download Presentation

On behalf of the

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Higgs Searches at Tevatron MaximTitov, CEA Saclay, France On behalf of the COLLABORATIONS 18 August 2011, Moscow, Russia

  2. Reaching the Higgs Horizon • Introduction • Challenges and Analysis strategies • Standard Model HiggsSearches • New Tevatron combined result • Beyond Standard Model Higgs The excellent performance of the Tevatronduring last few years has sparked the realisationthat a HiggsmightbeobservedatTevatron, thusensuring acomplimentarity of Tevatron and LHC Searches

  3. Exciting interplay of Higgs physics and direct supersymmetrysearches Self-consistency of the SM and Higgs Boson A light Standard Model Higgs hypothesis is in agreement with all indirect tests Indirect constraints • Precision electroweak observables are sensitive to the Higgs boson mass via quantum corrections. mH< 161 GeV (95% CL)

  4. The absence of a light Higgs implies New Physics beyond SM First the Higgs Boson has to bediscovered !!! Is it a Standard Model or MSSM Higgs...? Mass: determines SM Higgsprofile Width/partial width/ couplings Spin and CP quantum numbers Higgs self-coupling Tevatron/ LHC LHC/ sLHC/ Linear Collider Higgs is a journey, not a destination

  5. Run I Run IIa Run IIb Bunches in Turn 6  6 36  36 36  36 s (TeV) 1.8 1.96 1.96 Typical L (cm-2s-1) 1.6 1030 1x1032 4.0 1032  Ldt (pb-1/week) 3 15-20 50-60 Bunch crossing (ns) 3500 396 396 # int./ crossing 2.5 2.5 7.0 Tevatron Accelerator World’s highest energy proton-antiproton collider

  6. Tevatron: Run II IntegratedLuminosity Results today using up to 8.5 fb-1 Expect > 10 fb-1analyzabledata by end of September 2011

  7. Tevatron Experiments: CDF & D0 • Two general purpose detectors: • Central tracking system embedded • in a solenoidal magnetic field: • Silicon vertex detector • Tracking chamber (CDF) • Fiber tracker (DØ) • Preshowers • Electromagnetic and hadronic calorimeters • Muon system Rapidity coverage: CDF Dzero Tracking 2.0 2.5 Calorimeter 3.6 4.0 Muon 1.0 2.0 B-field 1.4 T 2.0 T

  8. SM Higgs Boson Production at Tevatron • Main production mechanisms (115<mH<180 GeV): • Gluon fusion (gg  H): ~0.8-0.2 pb • Associated production (VH, V=W,Z): ~0.2-0.02 pb • Vector boson fusion (VBF): ~0.1-0.02 pbs

  9. SM Higgs Decays and Search Strategy at Tevatron mH < 135 GeV: VH (V=W,Z) production with H→bb decay mH > 135 GeV: gg→H production with H→WW→lnln decay • Analyze all decay channels to • achieve the best sensitivity (e.g.): • Direct: gg H  tt, gg • VBF: qqH  qqbb, qqH qqWW • ttH  l+jets, ttH  all jets •  Combine all CDF/DO channels ~ 400 -700 Higgs events produced with 10 fb-1

  10. Backgrounds to SM Higgs Production • QCD Multijets(data driven methods) •  Jets faking leptons, ETmiss from mismeasured jets • Z/g + jets (ALPGEN/PYTHIA, NNLO theory cross-section, data-based corrections to model pT(Z)) •  mismeasured jets or leptons yielding MET • W+jets, W+g (ALPGEN/PYTHIA, NNLO theory cross-section, data-based corrections to model pT(W) and/or data drivenmethods) • jets or g faking lepton • Diboson - WW, WZ, ZZ (PYTHIA, normalized to NLO cross-section; NLO correction for pT and di-lepton opening angle) • ttbar, single top (ALPGEN/PYTHIA, COMPHEP; normalized to NNLO)

  11. Main Low Mass Higgs Channels • gg H  bb • final state overhelmed by QCD • Main channel: associated • production WH / ZH with H→bb • Extremely challenging (requires • Excellent b-tagging, dijet mass • resolution, bkgds understanding) ZHl l bb WH  lnbb ZHnnbb WH→lnbb: lepton+MET+2 b-jets  Largest signal rate  Larger V+jets background • ZH→lnbb: MET+2 b-jets •  Comparable signal rate to WH • (+WH→lnbb with missing lepton) • Challenging instrumental • background ZH→llbb: dilepton+2 b-jets Smallest Higgs signal rate  Low background  Kinematically constrained

  12. Searching for VH (H  bb) Step 1: Identify events consistent with leptonic W/Z decays and >= 2 jets • Trigger on high pT electrons, muons or ETmiss • Wln: e or m and high ETmiss • Zll: ee or mm consistent with Z resonance • Znn: no charged leptons; high Etmissand 2 acoplanar jets

  13. b-Jet Identification and Tagging Step 2: b-tagging (reduces backgrounds by two orders of magnitude) • B-tagging exploits information on: • Lifetime: displaced tracks and/or vetices • Mass: secondary vertex mass • Soft leptons • Use MVA for improved performance: •  NN for b-to-c discrimination after secondary vertex tagging; •  NN for b-to-light: continuous tagger(multiple operating points) b-jet eff. ~ 50% mis-tag rate ~ 0.5% e.g. DO: NIMA620 490-517 (2010) Before b-tagging =1 tightb-tag ≥2 looseb-tags S:B ~ 1:4000 S:B ~ 1:400 S:B ~ 1:75 13 13 13

  14. VH (H  bb): Control Regions Step 3: Validate background modelling in control regions Multijetenhanced: looseningmissingETmiss (and related variables) Top enhanced: require isolated lepton and two 2b-tag jets W+jets enhanced: require isolated lepton Similar control regions for other final states and heavy flavour enhanced samples

  15. Dijet Mass Resolution • The invariant mass of the bb pair is the most sensitive variable to the Higgs • An improvement in resolution has a direct impact on the search sensitivity Exploit information from tracker, preshower, jet shape variables, semileptonic b-decays with the NN  15% resolution improvement How to choose 2jets from 3jets or more?Instead of using two largest pT jets, use two most b-like jets from bID information. S/B remainssmall, needadvanced (multivariate) analysis techniques

  16. Multivariate Analysis Techniques Step 4: Optimize separation via multivariate technique • Exploit information from severaldiscriminantvariablesand their correlations • Improves sensitivity compared to cut-based analysis by ~15-20% • However, must be very careful with the choice of training sample • Many checks performed in different kinematic regions to validate • the modeling of the inputs to the MVA method and its output; • Same optimization/techniques in similar • final states as for Higgs searches: • Single top • Dibosonhadronicdecays

  17. DibosonHadronicdecays: Validation of Search Techniques WZ+ZZnnbb,nncc: VS D0 NOTE-6223 (2011) • For mH=115 GeV • WH→lbb: σ = 26 fb • ZH→bb: σ = 15 fb • ZH→llbb: σ = 5 fb • Total VH: σ = 46 fb • Replace Z with H • WZ→lbb: σ = 105 fb • ZZ→bb: σ = 81 fb • ZZ→llbb: σ = 27 fb • Total VZ: σ = 213 fb 17 17 17

  18. Interpreting the Data: Limit Plots • Use the final discriminant distribution (e.g. NN output) to perform • hypothesis testing (S+B vs B-only) • In the absence of excess, set limits using: •  A Bayesian method (flat prior signal, credibility intervals) •  The CLS method (log-likelihood test statistic CLS = CLS +B /CLB) Upper cross section limit for Higgs production relative to SM prediction Observedlimit (solid line) from data Expectedlimit(dot dashed line) and predicted1σ/2σ (green/yellow bands) variations from background only pseudo-experiments

  19. VH (H  bb): Low Mass HiggsSearches ZHllbb WHlvbb VHvvbb (7.9fb-1) (7.5fb-1 ) (7.8fb-1) CDF NOTE-10593 (2011) CDF NOTE-10583 (2009) CDF NOTE-10596 (2011) CDF NOTE-10572 (2011) Exp (obs) - 3.9 (4.8) x SM @ MH=115 GeV Exp (obs) – 2.7 (2.6) x SM @ MH=115 GeV Exp (obs) – 2.9 (2.3) x SM @ MH=115 GeV

  20. VH (H  bb): Low Mass HiggsSearches ZHllbb WHlvbb VHvvbb (8.6fb-1) (8.5fb-1 ) (8.4fb-1) D0 NOTE-6223 (2011) D0 NOTE-6166 (2011) D0 NOTE-6220 (2011) Exp (obs) – 4.8 (4.9) x SM @ MH=115 GeV Exp (obs) – 3.5 (4.6) x SM @ MH=115 GeV Exp (obs) – 4.0 (3.2) x SM @ MH=115 GeV

  21. Low Mass Higgs Searches: Summary 95% CL Limits at mH = 115 GeV: Tevatron Combination H  gg: D0: arXiv: 1107.4960

  22. Searches for High Mass Higgs Dominant decay mode for mH> 135 GeV: H WW Clean environmentcantake advantage of gg → H production: • 2 opposite charge highpT leptons • Missing ET (E Tmiss) Signal contribution alsofromassociated production (W/Z+H) and VBF (qqH): ~ 35 % more signal  Consider all final states with 2 high-pT leptons and ETmiss

  23. H  WW  lnln: Analysis Strategy Step 1: Preselect events with two isolated high-pT leptons • Split analysisaccording to: • D0: Lepton flavor: ee, em, mm • CDF: Signal purity based on lepton quality • CDF: Low (<16 GeV) di-lepton mass •  Different instrumental/fake backgrounds • Different background compositions Suppress the dominant Z/g background:  Use kinematics, in particular ETmiss based variables that ensure ETmiss is significant and not due to mis-measured object.  DO (ee, mm) employs Decision Trees trained against Z/g

  24. H  WW  lnln: Control Regions Step 2: Validation of background modelling and search techniques that share characteristics of the signal Diboson cross section measurements: W+jets : same-sign dileptons CDF NOTE-9753 (2009) Define control regions to test modelling for different backgrounds: t-tbar : opposite- sign dileptons, >= 2 jets, b-tag CDF NOTE-10358 (2010)

  25. H  WW  lnln: Final Discrimination Step 3: Multivariate analysis • Split analysis according to jet multiplicity: • better sensitivity to H+jets final states: qqH, WH, ZH (important for low mass) • Each multiplicity bin correspond to • a different dominant background: • 0 jet: WW • 1 jet: WW; Z/g • ≥2 jets: ttbar 2-jet 1-jet MVA optimized for each channel & mass hypothesis. Input MVA variables (e.g. D0): mH = 165 GeV 0-jet D0 NOTE-6219 (2011) CDF NOTE-10599 (2011)

  26. VH  V W+W-  l+l+ + X Additional sensitivity ( ~ 10%) from same charge dileptonselection • Main backgrounds are instrumental: • Lepton charge mis-ID (Z/g*l+l-) • Jets faking leptons (multijet, W+jet/g) Final multivariate (BDT, NN) discriminants to analyze data Exploit: Event topology, lepton kinematics, jet content, relation between lepton and ETmiss … W/Z W/Z D0: arXiv 1107.1268 (2011) CDF NOTE-10599 (2011)

  27. SystematicUncertainties: H  WW • Data consistent with the background-only hypothesis within the systematic uncertainties. • Significant sensitivity at high mass!

  28. TevatronCombination: July 2011 CDF Combination: D0 Combination: D0 NOTE-6229 (2011) Tevatron (CDF + D0) Combination: S+B versus B-onlyHypotheses (LLR = -2lnQ, where Q = Ls+b/Lb) Most “signal-like’ excess consistent with Higgs of 130 GeVbut also consistent with background-only hypothesis

  29. TevatronCombination: July 2011 Tevatron Combination: mHLimit/ SM (GeV)OBS. EXP. 115 1.22 1.17 130 2.02 1.37 165 0.48 0.58 180 1.17 1.98 Observed limit (data) July 2011 Tevatron Combination: arXiv:1107.5518 SM Higgs excluded @ 95 % CL: Observed Exclusion: 100 < mH < 108 and 156 < mH <177 GeV Expected Exclusion: 100 < mH < 109 and 148 < mH < 180 GeV

  30. Tevatron Projections and Prospects • Including ongoing analysis • improvements and more • channels: • Exclusion potential for • mH < 190 GeV • 2-3 s sensitivity for • mH ~ 115-130 GeV • Best current limit for mH<130 GeV • Unique window into H bb • H WW analysis sensitive to • different signals and backgrounds than LHC around 130-140 GeV 1-3 * SM

  31. Beyond the Standard Model Higgs

  32. Higgs Sector in MSSM • MSSM requires exactly 2 Higgs doublets: • one couples to up-type quarks (vev vu) • another couples to down-type quarks (vev vd) • Important parameter: tan b = vu/vd • tan b ~ 35 = mt/mb is appealing (large tan b) • After EW breaking: 5 physical states • ‣ 3 neutral Higgs bosons: h/H (CP-even) • and A (CP-odd) • (convention: mh < mH, h/H/A generically denoted j) • ‣ 2 charged Higgs bosons: H± • At tree level: EW breaking controlled by MA • and tanβ. Radiative corrections make it • more model dependent. • • There must be a light Higgs (h): mh ≤ 135 GeV H/A/H+ nearly equal mass when mAlarge Higgscouplingto b-quarks enhanced by tan β  sPROD ~ tan2b

  33. Neutral MSSM Search Strategy: h/H/A Decay Modes Overwhelming QCD background • Relatively clean signature • low BR ~10% Three complimentary channels: • High BR ~90% • Large multijet background • Reduced background • Additional sensitivity at low mA

  34. BSM Higgs: b(b) + F0 bbb(b) • Both CDF and DØ see ~2s excesses around mA~120-150 GeV CDF: arXiv: 1106.4782 (2011) D0: PLB698, 97 (2011)

  35. BSM Higgs: b(b) + F0 tt and bF  b tt • Tevatron searches does not observe any significant excess ftt arXiv: 1106.4555 (2011) D0 Note 6227 (2011) bfbtt

  36. Summary & Outlook • CDF and D0 have paved the way and brought sophistication and maturity into Higgs boson searchesat hadron colliders. • Tevatron is on track to deliver Higgs search results in spring 2012 based on the full 10 fb-1 datasets with promised sensitivity goals NATURE WILL, IN ALL LIKELIHOOD, SURPRISE US !

  37. Back-Up Slides

  38. What Would a Higgs Signal Look Like Signal Injection Test (6 fb-1): Tevatron Observed Limit: • Consider main low mass analyses (WHlnbb, ZHnnbb, ZHllbb) at 6 fb-1 and evaluate expected LLR after injecting a SM-like signal at mH=115 GeV •  observed limit consistent with a what would be expected from signal+background (but also consistent with background-only) A. Juste, 2011 DPF Meeting, August 2011

  39. ATLAS and CMS Combinations

More Related