1 / 27

Higgs Searches at DØ

Higgs Searches at DØ. Marcel Demarteau Fermilab For the DØ Collaboration. LCWS06, Bangalore, India March 9-13, 2006. Tevatron Run I: 1992 - 1996 ∫ L dt = 120 pb -1 E CM = 1.8 TeV Tevatron Run IIa: Δ t = Δ t(Run I) (2001 - 2006) ∫ L dt = 10 * ∫ L dt (Run I) E CM = 1.96 TeV

steffie
Download Presentation

Higgs Searches at DØ

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 DØ Marcel Demarteau Fermilab For the DØ Collaboration LCWS06, Bangalore, India March 9-13, 2006

  2. Tevatron Run I: 1992 - 1996 ∫ L dt = 120 pb-1 ECM = 1.8 TeV Tevatron Run IIa: Δt = Δt(Run I) (2001 - 2006) ∫ L dt = 10 * ∫ L dt (Run I) ECM = 1.96 TeV Tevatron Run IIb: Δt < Δt(Run IIa) (2006 - 2009) ∫ L dt ≈ 5 * ∫ L dt (Run IIa) ECM = 1.96 TeV Outline: Introduction Low mass Higgs search High mass Higgs search MSSM Higgs search Prospects Tevatron Experimental Program LCWS06, Bangalore, March 2006, Marcel Demarteau

  3. Constraints on Higgs Mass in SM • Limit from direct searches at LEP: mH > 114.4 GeV (95% CL) • Indirect limit from fits to precision EW data from LEP, SLC and Tevatron • mH < 219 GeV (95% CL) with • mt = 172.7 ± 2.9 GeV (CDF, D0, Run I+II) • Indirect best fit value • mH = 91 +45-32 GeV LCWS06, Bangalore, March 2006, Marcel Demarteau

  4. SM Higgs Boson Production and Decay • Dominant Decays: • Low mass: bb; High mass: W+W- • Search strategy: • MH < 135 GeV: associated production • qq → WH / ZH production, with H → bb • Dominant backgrounds: Wbb, Zbb and tt • MH > 135 GeV: direct production • gg → H (or WH), with H → WW* • Dominant backgrounds: WW and WZ production σ (pb) Excluded at LEP LCWS06, Bangalore, March 2006, Marcel Demarteau

  5. Search Channels for SM Higgs • Notes: • WH channel in leptonic modes with b-tag • ZH channel all hadronic decay mode • WW exploit scalar nature of Higgs • WWW like-sign di-leptons LCWS06, Bangalore, March 2006, Marcel Demarteau

  6. - WH → enbb • Event selection: • central electronpT > 20 GeV, |h|<1.1 • veto second lepton • Missing ET > 25 GeV • Two b-jetspT > 25 GeV, |h|< 2.5at least one 1 b-tag (impact parameter b-tag) • Backgrounds: • W+jets, multi-jet, tt, single top, WZ • Single b-tag sample dominated by background: • Used as control sample • Limit extracted from double b-taggedsample using di-jet mass distribution inclusive (≥1) b-tag LCWS06, Bangalore, March 2006, Marcel Demarteau

  7. - WH → enbb • Limit set from di-jet invariant mass spectrum • 4 events observed in mass window 85 GeV < mbb <135 GeV • 2.37 ± 0.59 events expected background • Thus, upper limit on WH cross section of 8.6 pb. LCWS06, Bangalore, March 2006, Marcel Demarteau

  8. - - ZH → nnbb • Event rate: s · BR  0.01 pb  s(WH → ℓnbb) • Event selection: • Missing ET > 25 GeV • 2 aco-planar b-jets, ET > 20 GeV, |h| < 2.5 • b-tagging • lifetime probability algorithm: jets having tracks with large impact parameters (JLIP) • B-tagging efficiency ~43% with 0.5% mistag rate after quality cuts • Backgrounds: • Physics: W+jets, Z+jets, top, WZ, ZZ • No isolated track, HT < 200 GeV • HT defined as the scalar sum of jets’ ET • Instrumental: multijets w/mistag • Jet aco-planarity, various asymmetries • Strategy • Trigger on events with large missing HT • Estimate “instrumental” bckg. from data • Search for an event excess in di-b-jet mass distribution ≥ 0 b-tag LCWS06, Bangalore, March 2006, Marcel Demarteau

  9. Signal MC Data - - ZH → nnbb: Background Treatment • Definition of various missing energy / momentum variables • Form various asymmetries • For signal events nn and bb are balanced: asymmetries peak at 0. • Measure instrumental background from the sidebands ET HT n n ET for mis-measured jet Jet2 Jet1 LCWS06, Bangalore, March 2006, Marcel Demarteau

  10. - - ZH → nnbb • Set limit on cross section from observed number of events in double tagged sample as function of Higgs mass For MH = 115 GeV bin • Systematic uncertainty • 26% on signal acceptance • 33% on background • dominated by uncertainty on b-tag eff. LCWS06, Bangalore, March 2006, Marcel Demarteau

  11. H → WW* → ℓ+ ℓ- nn • Dominant production mechanism for higher mass Higgs • 3 decay channels, oppositely charged leptons: ee, em, mm • Leptonic final state allows for excellent event modeling • Event Selection: • pT > 15 (10) GeV for leading (trailing) lepton • missing ET > 20 GeV • sum of pT of leptons and missing ET • HT < 100 GeV • Backgrounds: • WW, WZ, ZZ, Drell-Yan, W+jets, W+g, tt, multijets • Z-events removed through Df(ℓℓ) cut • tt-events removed through HT cut ee em mm LCWS06, Bangalore, March 2006, Marcel Demarteau

  12. H → WW* → ℓ+ ℓ- nn • Opening angle between leptons (∆fℓℓ<2.0) is useful discriminant • Two leptons from Higgs tend to move in parallel due to scalar nature of Higgs • Limit set on s · BR for all three channels combined as function of Higgs mass LCWS06, Bangalore, March 2006, Marcel Demarteau

  13. WH → WWW* → ℓ± ℓ± nn qq • Select like-sign isolated leptons • one of W’s from Higgs decay has same-sign lepton as associated W • Event Selection: • Two isolated e/m pT > 15 GeV • Third lepton veto • Missing ET > 20 GeV • Backgrounds largely avoided • Physics: • WZ production • Instrumental: • dominant W+jets • “charge flips” accounted for by estimating flip probability from data: ratio of like to unlike sign events at high invariant mass (Mll>70 GeV) • Pflip = (9.7 ± 3.1) 10-4 (ee) • Pflip = (11.7 ± 2.6) 10-4 (mm) LCWS06, Bangalore, March 2006, Marcel Demarteau

  14. WH → WWW* → ℓ± ℓ± nn qq • “fermiophobic Higgs“ model • Br(H → WW) close to 100 % for Higgs masses down to ~ 100 GeV • L. Braucher, R. Santoshep-ph/9907434. LCWS06, Bangalore, March 2006, Marcel Demarteau

  15. Current Limits SM Higgs LCWS06, Bangalore, March 2006, Marcel Demarteau

  16. Limits / SM LCWS06, Bangalore, March 2006, Marcel Demarteau

  17. MSSM Neutral Higgs Search • In MSSM three neutral Higgs particles predicted, h0, H0, A0 • At tree level Higgs coupling to down-type quarks, i.e. b-quarks, is enhanced with respect to the SM, proportional to tanb • Production cross-section rises as tanb2 • Larger s for part of parameter space than SM • Search assumptions: • No difference between A0 and h0/H0 • Mass degeneracy • 100-130 GeV: h0, H0, A0 • higher mass: h0, A0 or H0, A0 • Total signal cross-section twice that of the A0 boson • Search strategy: • multi-jet high ET sample • 3 or more jets b-tagged J. Campbell, R. Ellis, F. Maltoni, S. Willenbrock Alternate, consistent, calculation by: S. Dawson, C. Jackson, L. Reina, D. Wackeroth LCWS06, Bangalore, March 2006, Marcel Demarteau

  18. MSSM Neutral Higgs Search • At least 3 tagged b-jets; Look for excess in di-jet mass spectrum • Signal rates and kinematics are normalized to NLO calculations • Background shape determined from double b-tagged data, applying tag rate function to non-b-tagged jets • Two scenarios considered • No mixing in stop sector: Xt = At – mcotb = 0 (m = – 0.2 TeV) • Maximal mixing: Xt = √6×MSUSY, MSUSY= 1 TeV Fitting outside signal region (±1s of peak) LCWS06, Bangalore, March 2006, Marcel Demarteau

  19. Development of new Neural-Network b-tagging algorithm which yields 34% increase in b-jet ID efficiency for the same fake rate Construction of new small radius silicon detector, with expected improvement of impact parameter resolution of ~50% for 10 GeV pT Prospects • Layer 0 is being installed as we speak ! LCWS06, Bangalore, March 2006, Marcel Demarteau

  20. Summary • Full range in mass of Higgs boson being explored in many different channels • Current limits still far from Standard Model predictions • Significant progress anticipated from: • Analysis of full data sets • Refinements in current analyses and development of new analysis tools • Combination of all different search channels • Combination of results from two collider experiments • Exploitation of new silicon, Layer 0, detector ! LCWS06, Bangalore, March 2006, Marcel Demarteau

  21. Backup LCWS06, Bangalore, March 2006, Marcel Demarteau

  22. - - ZH → nnbb • For mH = 115 GeV, expect 2.19 events in di-jet mass window 80 GeV < Mjj < 130 GeV • observe 3; thus cross section limit of 9.3 pb at 95% confidence level • Systematic uncertainty dominated by uncertainty on b-tagging efficiency: • 26% on signal acceptance • 33% on background 1 btag 2 btags 0 btag LCWS06, Bangalore, March 2006, Marcel Demarteau

  23. ZH → nnbb: Before b-tagging Total Data : 2140 Expect : 2125 LCWS06, Bangalore, March 2006, Marcel Demarteau

  24. ZH → nnbb: Single b-tag Total Data : 132 Expect : 145 LCWS06, Bangalore, March 2006, Marcel Demarteau

  25. ZH → nnbb: Double b-tag Total Data : 9 Expect : 6.4 LCWS06, Bangalore, March 2006, Marcel Demarteau

  26. Prospects LEP • Updated in 2003 in the low Higgs mass region • better understandingof detector • optimization of analyses • Sensitivity in the mass region above LEP limit starts at ~2 fb-1 • Current results show that optimal sensitivity, as assumed by past Higgs working group studies, has not yet been reached (typically by factor 2-3) • selection efficiency • b-tagging efficiency • trigger efficiency • larger backgrounds • mass resolution • …… ∫Ldt, fb-1 Tevatron • But, … LCWS06, Bangalore, March 2006, Marcel Demarteau

  27. Impact Parameter Resolution LCWS06, Bangalore, March 2006, Marcel Demarteau

More Related