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Search for the Standard Model Higgs Boson at D Ø

Search for the Standard Model Higgs Boson at D Ø. Michele Petteni Imperial College London On behalf of the DØ Collaboration. Talk Outline. What do we expect from the Standard Model? Higgs production and decay at the Tevatron Expectations for the Tevatron

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Search for the Standard Model Higgs Boson at D Ø

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  1. Search for the Standard Model Higgs Boson at DØ Michele Petteni Imperial College London On behalf of the DØ Collaboration

  2. Talk Outline • What do we expect from the Standard Model? • Higgs production and decay at the Tevatron • Expectations for the Tevatron • Brief description of DØ and the Tevatron • Analysis channels • Outlook

  3. Theoretical limits: WLWL scattering, mH< 1 TeV Other limits depend on the scale of new physics Higgs boson needed for Electroweak symmetry breaking: Allows for difference in W,Z and photon mass Generates masses for fermions Neutral, scalar particle The Higgs Boson

  4. Experimental Constraints • Higgs couples to mass : • W and Top mass measurements constrain mass of Higgs via effect of radiative corrections. • Global Electroweak fits give indication of Higgs mass • Recently updated for new top mass measurement from Tevatron. • Direct Searches • This has not changed from LEP limit, 114.4 GeV

  5. Higgs Production and Decay Excluded by LEP • Two search strategies: • Below 135 GeV look for associated production with 2 bjets from the Higgs decay. • Above this limit exploit gluon fusion and reconstruct the two gauge bosons. • Possibility of third strategy in the intermediate mass range? • More on this later

  6. Sensitivity studies performed : Higgs Working Group Collaboration,hep-ph/0010338 Tevatron Higgs Sensitivity Study Group, FERMILAB-PUB-03/320-E Studies have some inherent assumptions For low mass Higgs : Excellent b-tagging to identify jets from the Higgs Lepton-id to identify the Z/W Jet resolution is essential to disentangle the mass peak For high mass Higgs : Hinges around lepton identification Sensitivity studies highlight important facts Not one golden channel Combined results from both Tevatron experiments Evidence for light mass Higgs is feasible at the Tevatron Why the Tevatron? 8 fb-1 for Run IIb

  7. The DØ Detector

  8. Operations Report • Luminosity progressing steadily and in a regular manner • 1 fb-1 delivered • DØ operating well and is stable • Analysis presented taken with data up to latest shutdown Peak luminosity > 1.2x1032 Data used in analysis Stable at 88% efficiency

  9. Experimental Results • Low mass Higgs : • Z/* ( e+e-) + ≥ n jets x-section • WH  e  b b • ZH    b b • Intermediate mass Higgs : • WH  WWW* • High Mass Higgs • H  WW*

  10. Z/* ( e+e-) + ≥ n jets • Z/W + n jets main background to light mass higgs searches • Main cuts: • Two leading electrons pT > 25 GeV, consistent with Z mass • Jet pT > 20 GeV, || < 2.5 • Results normalised to inclusive cross-section: • Jet multiplicity figure compared with NLO MC and to matrix-element - parton shower treatment (ME-PS) • Jet pT spectrum compared to Alpgen + Pythia normalised to data

  11. Cross-section Ratios • Next step measure Z+n bjet cross-sections. • Higgs limit

  12. WH Searches; W(e)+jets • Results are an update on the analysis published in PRL 94, 091802(2005). • Includes data shown in the PRL paper (174 pb-1) • WH process has a higher x-section than ZH and benefits as only one lepton to identify • Main backgrounds are Wjj (before b-tagging) and subsequently tt, single top, and WZ and the ever present QCD

  13. WH Searches; W(e)+jets (2) • Main cuts: • Isolated electron, pT > 20 GeV, || < 1.1 • Missing ET > 25 • Two jets, pT > 20 GeV, || < 2.5 • After basic selections 3844 events, expect 3256 W/Z+jets events • Good agreement with MC

  14. WH Searches; W(e)+jets (3) • After b-tagging see 13 events expect 10.2 • B-tagging efficiency major source of uncertainty • Set limit using a mass window for 4 Higgs mass values

  15. ZH bb • Due to the large branching ratio for Z decay this channel has about the same contribution as all the W channels combined • However : • No “easy” handle to trigger the events • trigger on missing ET from the jets and the presence of jets • Highly sensitive to any mis-measurements • Significant QCD background • large cross-section • difficult to estimate • Main non-QCD backgrounds are Zbb, ZZ, ZW, Wbb and top (after b-tagging)

  16. ET HT PTtrk PT.2trk Jet1 Jet2 Topology Cuts • Require two acoplanar jets, missing ET and veto any isolated leptons • In order to reduce the QCD/instrumental background, exploit correlation between missing energy from calorimeter cells, jets and tracks • They should be aligned • Form asymmetries and cut on this value (peaks at ~0 for signal) • Cut on angles between jets and missing ET

  17. Data Instrumental bkgd. from sidebands Double Gaussian Exponential Estimation of Instrumental Background sideband sideband Physics bkgd. from MC

  18. ZH bb Results • Expect 2125, see 2140 before b-tagging. • After b-tagging observe 9 expect 6.4 • Apply 50 GeV mass window • Signal acceptance 0.3% • Zbb and Wbb major backgrounds • Instrumental half of these (same as top)

  19. WHWWW*llqq • Exploits like-sign leptons in final state • Low backgrounds • Clear signal • Require 2 like sign leptons (veto 3rd) • Missing ET > 20 GeV • Main backgrounds • Physics, WZlll • Charge flips from Zll, WWll,ttll, etc. • Instrumental from “real” like sign leptons from semileptonic decays,  conversions...

  20. WHWWW*llqq (2) • Different signals have different background contributions • Observed limit covers nicely the intermediate Higgs mass region

  21. HWWll • Look in ee,  and e channels • Require: • Two opposite sign leptons • Missing ET > 20 GeV • Cut out events with potentially large contribution to the missing energy from jet mis-measurement • Veto low mass resonances and reduce Z contribution Excellent agreement after basic cuts

  22. WHWWll (2) • In order to further suppress background use angular correlations: • Spin of Ws are correlated • Leptons from Higgs are collinear, back-to-back for background • Cut on opening angle between ll • Mass dependent cuts Selection acceptance varies from 16.7% to 3.9% Combine all channels using likelihood functions Numbers of events depends on Higgs mass, for mH= 160 GeV expect 17.7±1.0 and see 20.

  23. Summary • Preliminary results look good • Good description of background processes and the understanding of environment has improved dramatically • Analysis (always) need more work, striving to reach sensitivity outlined by sensitivity study • Need advanced analysis techniques • Tevatron has already delivered 1 fb-1 (0.8 fb-1 on tape), by the end of the year we will have 1 fb-1 to tape • With this luminosity getting close to excluding at 95% a Higgs mass close to 115 GeV

  24. Backup Slide

  25. Comparison of WH published Results with Sensitivity Prospective Study We are currently missing a factor 2.4 in sensitivity Prospective Study assumed: Larger ECAL coverage (+30%), improved em-id (+40%), extended b-tag efficiency (+50%, 2 tags) and 30% less backgd (better dijet mass resolution)  Factor 2 in S/B  2.4/ 2 = 1.2 difference (only) in sensitivity Apply Advanced techniques Missing Factors can be recovered

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