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D0-France, October 14 th 2008 Gregorio Bernardi, LPNHE-Paris for the CDF and D Ø Collaboration

WH, High Mass, Combined Higgs Searches and Prospects at the Tevatron. Introduction Standard Model Higgs Searches Combination techniques Combination results: Prospects. D0-France, October 14 th 2008 Gregorio Bernardi, LPNHE-Paris for the CDF and D Ø Collaboration.

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D0-France, October 14 th 2008 Gregorio Bernardi, LPNHE-Paris for the CDF and D Ø Collaboration

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  1. WH, High Mass, Combined Higgs Searches and Prospects at the Tevatron • Introduction • Standard Model Higgs Searches • Combination techniques • Combination results: • Prospects D0-France, October 14th 2008 Gregorio Bernardi, LPNHE-Paris for the CDF and DØ Collaboration

  2. SM Higgs boson production • WH, ZH associated production • Important at hadron colliders since can trigger on 1 or 2 high-pT leptons, Jets and MET • t t H (and bbH) associated production • High-pT lepton, top reconstruction, b-tag • Low rate at the Tevatron H • gg fusion • Dominates at hadron machines • Usefulness depends on the Higgs decay channel • Vector Boson Fusion • Two high-pT forward jets help to “tag” event • Important at LHC, first results at Tev!

  3. SM Higgs Production and Decays Decays Production Production cross section (mH 115-180)  in the 0.8-0.2 pb range for gg  H  in the 0.2-0.03 pb range for WH associated vector boson production Dominant Decays  bb for MH < 135 GeV  WW* for MH > 135 GeV

  4. SM Higgs Low mass searches: datasets/methods ZHnnbb ZHl l bb WH Lepton Missing ET 2 b-jets Missing ET 2 b-jets 2 Leptons 2 b-jets CDF DØ CDF DØ CDF DØ Prel: 2.7 fb-1 1.7 fb-1 2.7 fb-1 2.1 fb-1 2.4 fb-1 2.3 fb-1 Pub: 1.0 fb-1 1.1 fb-1 1.0 fb-1 0.9 fb-1 1.0 fb-1 0.4 fb-1 Common to all analyses: b-tagging, Jet calibration & resolution, lepton-identification, Background cross-section Differences: instrumental bckd, multivariate techniques

  5. SM Higgs Searches @ Tevatron: WHlbb l+ W+ Signal u t u l+ l u W+ d Large background (also Wcc) b l t d b H0 W*+ “ttbar”: Jets+leptons from W decay b d b l+ W+ u u q l d W+2 light jet with one/two false b-tag q d l+ W+ u “Non-W from QCD” t l b W+ l+ W*+ and “Non-W from EW”, e.g. all Zbb processes In which one lepton is lost b u l d Single Top d b Z0 “Diboson” b

  6. Selecting Wlnbb Select events using W decay signatures Require one high-pT leptons: pT > 15 GeV) Neutrinos  missing transverse energy WHlnbb:MET > 20 GeV Reconstruct vector boson mass Use “OR'ing” of muon triggers: 100% efficiency • +15% in sensitivity. Single EM or EM+jets for e-channel (95% eff) Select >=2 high-pT, central jets as a first step towards a Higgs signature pT>20 GeV,|eta|<2.5 Reweight W+jets ALPGENto data in eta, phi, delta-eta and delta-phi of jets, before b-tagging

  7. Neural Network b-tagger LOOSE (eff=70%, fake=4.5%) Vertex Tagging (transverse plane) All Higgs analyses uses Neural Network b-tagging algorithm, Combining best aspects of secondary vertex algorithm statistical impact parameters algo Asymmetric tagging: Tight tagging for Single Tag Loose tagging for Double Tag • Large improvement compared to the individual taggers: Loose 72% b-tagging eff.6% mistag Tight  50% b-tagging eff. 0.3% mistag B (Signed) Track Decay Lengh (Lxy) Impact Parameter (dca) Secondary Vertex Tagger Jet LIfetime Probability Counting Signed Impact Parameter Hard Scatter NNbtag Gain > 30% compared to our individual taggers (secondary vertex or impact parameter)

  8. WHl bb (l=e,): effect of b-tagging Starting from a W+ 2 jet selection, apply NN_btagging: Pre-tag Exclusive single tag (Tight) Dijet mass, GeV Dijet mass, GeV

  9. WHl bb (l=e,): after b-tagging Starting from a W+ 2 jet selection, apply NN_btagging  orthogonal samples Exclusive single tag Double Loose tag Higgs x10 Dijet mass, GeV Dijet mass, GeV Backgrounds are measured one after the other (Wbb, Single Top), WZ with Zbb remains the golden benchmark on which we can tune our analysis tools.

  10. Use neural network to separate signal from background Fit the NN output WHl bb (l=e,): Neural Net and Limits pT(j1) pT(j2) R(jj) (jj) pT(jj) M(jj) pT(l,MET) NNwh Higgs x10 Future improvements (short term): include forward electrons and 3 jets sample. Improve NN with more backgd rejection and use Matrix Element approach

  11. Summary of WH published Results PRL’05: S. Beauceron’s Ph.D thesis PLB’07: L. Sonnenschein’s Habilitation PRL’08: J. Lellouch’s Ph.D thesis In the making: 4 fb-1 result / N. Huske’s future thesis (’10) 7 fb-1 result/ J. Brown’s future thesis (’11)

  12. ICHEP Preliminary WHlbb • WHlbb - signature: high pT lepton, MET and b jets • Backgrounds: W+bb, W+qq(mistagged), single top, Non W(QCD) • Key issue: estimating W+bb background • Shape from MC with normalization from data control regions • Innovations: CDF: 20% acceptance from isolated tracks, NN jet corrections TWO approaches Neural Net and Matrix Element+Boosted Decision Trees Working on combining both for Tevatron combination. Results at mH = 115GeV: 95%CL Limits/SM

  13. Other SM Higgs Searches • CDF and DØ are performing searches in every viable mode: • CDF: VHqqbb: 4 Jet mode. • CDF: H with 2jets • Simultaneous search for Higgs in VH, VBF and ggH production modes • Interesting benchmark for LHC • DØ: H  • Also model independent and fermiophobic search • DØ: WHbb, new mode • Dedicated search with hadronic  decays • DØ: ttH, new mode

  14. SM Higgs Searches at Higg Mass at the Tevatron Low mass (mH <~135 GeV): dominant decay: Use associated production modes to get better signal/background Intermediate mass: High mass (mH>~135 GeV): dominant decay: VBF also contributes ~ 6%

  15. Search for WH  WWW* in di-lepton mode Same charge di-lepton from W’s (one from HWW, the other from prompt W) Data (Signal@ 160 GeV) 19 (0.10) 15 (0.21) 5 (0.11)  Topological Likelihood discriminant, built on 3 variables per channel (ETmiss, Df,’s) Limits ~20 times higher than SM at 160 GeV  useful in the intermediate region 125-140 GeV .. and also for fermiophobic Higgs searches

  16. High Mass: gg ➙H ➙WW*➙lnln(l=e,mu)

  17. H  WW*  lnln W + jet/g production: • Selection Strategy: • Presection:lepton ID, isolation,trigger, opposite charge leptons • Remove QCD and Zl+l-:ET > 20 GeV • Higgs Mass Dependent Cuts:Invariant Mass (Ml+l-); Min. Transverse Mass Sum of lepton pTl and ET (S pTl + ET) • Anti tt(bar) cut: HT = S PTjet < 100 GeV, or less than 2 jets • Spin correlation in WW pair:Df(l,l) < 2.0 • Then apply advanced analysis technique, Neural Net+Matrix element Major backgrounds WW production Now measured at the Tevatron by both expts. in agreement with NLO calculation: ~13.5 pb

  18. H  WW*  lnln : Selection Signal MTmin/GeV Signal (x10)

  19. Neural Network Input Variables /Matrix Elements 5 Matrix Elements discrim. H

  20. Matrix Elements + Neural Net  Final Discriminant Matrix elements discriminant

  21. SM Higgs: HWW • Most sensitive Higgs search channel at the Tevatron Results at mH = 165GeV : 95%CL Limits/SM Both experiments Approaching SM sensitivity!

  22. Combining the results Systematics, including correlations, are taken into account when combining results.  Correlations of uncertainties of analyses inside one experiment (e.g. Jet calibration), or across experiments (e.g. PDF, theoretical cross-sections) 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 Flavor

  23. Limit Setting • LEP: low background, small systematics • Tevatron: high background, large systematics (at low mass) • But SMALL signals in both cases • Background only (b) and signal plus background (s+b) • hypotheses are compared to data using Poisson likelihoods. • For the searches and to set limits, Tevatron experiments use generalized CLs method (modified frequentist, DØ) and Bayesian methods (CDF), and cross-check each other. • Systematic uncertainties are included in the likelihood, via Gaussian smearing of the expectation (‘profile likelihood’). • New compared to LEP: • Background is constrained by maximising profile likelihood (‘sideband fitting’), useful in particular at low mass. Limit setting approaches agree to better than 10%

  24. Constraining Systematic Uncertainties with Data Background prediction + uncertainty AKA side band fitting Nuisance parameters introduced in the chi2 of the fit allow shifting of central value of the background estimation Systematic uncertainty width gets also constrained Shape of the systematic is also taken into account “Profiling”

  25. Best fit for nuisance parameters DØ Higgs SM combination Centered close to  good shift in units of systematic uncertainty Fitted Nuisance parameters for each systematic uncertainty No smoking gun after all the checks, proceed to derive combined limit…

  26. Combining the Results Channel Lumi /Technique Final state Total of 28CDF + DØ channels combined

  27. Post-Moriond 2008, with up to 2.4 fb-1 3.3  SM at mH=115 GeV 1.6 SM at mH=160 GeV arXiv:0804.3423 • Observed limit at mH= 160 Gev: 1.1 x SM(3.6 @115 GeV) • Very close to excluding a 160 GeV SM Higgs. @ ICHEP: ~ 3 fb-1

  28. SM Higgs Limits with 3 fb-1 • Limits calculating and combination • Using Bayesian and CLs methodologies. • Incorporate systematic uncertainties using pseudo-experiments (shape and rate included) (correlations taken into account between experiments) • Backgrounds can be constrained in the fit • Low mass combination difficult due to ~70 channels, not updated yet but Expected sensitivity of CDF/DØ combined: <3.0xSM @ 115GeV Combination of CDF and D0 done at high Mass > 150 GeV

  29. SM Higgs Combination at High Mass High mass only Expected: 1.2 @ 165, 1.4 @ 170 GeV Observed: 1.0 @ 170 GeV

  30. SM Higgs Combination at High Mass • Result verified using two independent methods(Bayesian/CLs) 95%CL Limits/SM SM Higgs Excluded: mH = 170 GeV We exclude at 95% C.L. the production of a SM Higgs boson of 170 GeV

  31. Projection assumptions: High mass Higgs (Dec. 07) • Since 2005, our high Higgs mass experimental sensitivity has improved by a factor of 1.7 (i.e. taking out gain due to luminosity) • NN discriminants • Lepton acceptance • For 2010, we estimate an additional improvement in analysis sensitivity by a factor of 1.4 • increased lepton efficiency (10% per lepton) • multivariate analyses (~30% in sensitivity) • Potential improvements not included in estimate • add  channels • …

  32. Sensitivity and Projections – MH = 115 GeV • Since 2005, our analysis sensitivity has improved by a factor of 1.7 wellbeyond improvement expected from sqrt(luminosity) • Acceptance/kin. phase space/Trigger efficiency • Asymmetric tagging for double b-tags • b-tagging improvements (NN b-tagging) • improved statistical techniques/event NN discriminant • for channel with largest effort applied (WH) factor was 2.1 • For 2010, we estimate that we will gain an additional factor of 2.0 beyond improvement expected from sqrt(luminosity) • b-tagging improvements • DØ: Layer 0 (~8% per tag efficiency increase) • add semileptonic b-tags (~5% per tag efficiency increase) • Di-jet mass resolution (18% to 15% in σ(m)/m) • increased lepton efficiency (10% per lepton) • improved/additional multivariate techniques (~20% in sensitivity)

  33. Expected Higgs sensitivity in 2009/2010 Assumes two experiments Projection: DØ X 2 2010 2009 2009 By the time LHC produces Physics (2009), precision EW meas. + Tevatron might allow SM Higgs only with mass between 118 and 145 GeV, definitely only a light Higgs boson, which will take some time to be found at LHC (> 1 fb-1) LHC/Tevatron complementarity H gg vs Hbb

  34. Higgs sensitivity, 3-σ evidence Projection: DØ X 2 Assumes two experiments 2010 2009 • With data accumulated by the end of 2010, we will be able to explore much of the SM Higgs mass region allowed by the constraints from precision measurements and LEP direct exclusion • Expected 95% CL exclusion over whole allowed range, (except possibly around 130 GeV) - assuming the Higgs does not exist at these masses • Three-sigma evidence for a Higgs possible over almost entire range, and probable for the low end and high end.

  35. Conclusion New Higgs analyses available, large common effort by both CDF and Dzero collaborations. Sensitivity constantly progressing ~linearly with Luminosity. The Tevatron experiments have achieved sensitivity to the SM Higgs boson production cross section at High Mass, still working on low Mass • 2009 will also teach us how well we perform at low mass, with the golden WZ/ZZ (Zbb) benchmark • The Higgs boson search is entering its most exciting era, since it is for the first time within reach in the very near future (< 3 years). • We exclude at 95% C.L. the production of a SM Higgs boson of 170 GeV • We expect large exclusion, or evidence, with full Tevatron data set and improvements Report for conseil scientifique In2p3, july 2000

  36. Expected

  37. Systematics: ZH-llbb / CDF vs DØ

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