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Search for the Higgs Boson in the h  gg Channel

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  1. Search for the Higgs Boson in thehggChannel APS-DPF 2003 Philadelphia Alex MelnitchoukBrown University on behalf of the DØ collaboration Alex Melnitchouk APS-DPF conference - 2003

  2. = 2 TeV (Z) (Z*) Leading SM Higgs Production Processes at Tevatron gluon fusion : cross-section ~ m2  the top-quark loop is dominant Cross-Section, pb 10.0 W/Z associated 1.0 0.1 0.01 80 100 120 140 160 W/Z fusion Higgs Mass, GeV Alex Melnitchouk APS-DPF conference - 2003

  3. hggdecay in the Standard Model • no tree-level Hgg coupling (Higgs is neutral) • diphoton decays happen via W or top-quark loop:  Hgg branching fraction is small: ~ 10-3-10-4 Higgs decays mostly to b-quark and W pairs • however many extensions of the SM predict enhanced gg decay rate of the Higgs Alex Melnitchouk APS-DPF conference - 2003

  4. hggdecays can be enhanced • due to suppressed couplings to •  all fermions (Fermiophobic Higgs)H.E.Haber et al, Nucl. Phys. B161, 493 (1979) • all fermions but top quark (Topcolor Higgs)B.Dobrescu, Phys. Rev. D63, 015004 (2001) • top and bottom quark J.D. Wells, Phys. Rev. D56, 1504 (1997) •  down-type fermions,H. Baer, J.D. Wells, Phys. Rev. D57, 446 (1998) • Enhanced ggrate is also predicted by some models that employLarge Extra DimensionsL. Hall, C.Kolda, Phys. Lett. B459, 213 (1999) • in addition gg decay loop can be enhanced by a contribution from a heavy charged particle coming from new physics Alex Melnitchouk APS-DPF conference - 2003

  5. Examples of Enhancement of hgg decays hggBranching Fraction no couplings to fermions (Fermiophobic Higgs) no couplings to top,bottom quarks no couplings to down-type fermions Standard Model Higgs Mass, GeV S.Mrenna, J.Wells, Phys. Rev. D63, 015006 (2001) in general we should be prepared for any hgg branching fraction ( up to 1.0 ) due to new physics Alex Melnitchouk APS-DPF conference - 2003

  6. hggSearch Strategy Focus on 2 Scenarios • Fermiophobic Higgs (does not couple to fermions) • Production: W/Z associated + W/Z fusion • Main signature with diphotons : gg + 2jets • Current Mass Limits : D0/CDF RunI : 78.5 / 82.0 GeV at 95% CL • LEP :108.3 GeV at 95% CLhep- ex/0212038 (2002) • Topcolor Higgs (of all fermions couples only to top) • Production: all three leading processes • Main signature with diphotons : gg • Remaining models would give similar signal to one of the two scenarios: e.g. no couplings to down-type fermions  topcolor no t,b couplings  fermiophobic; B.Abbot et al. Phys. Rev. Lett. 82, 2244 (1999 ) F.Abe et al. Phys. Rev. D59, 092002 (1999) LEP Current setting limits on Cross-Section  B(gg) Focus : for the Topcolor Higgs assuming SM couplings Alex Melnitchouk APS-DPF conference - 2003

  7. Major Backgrounds : Drell-Yan and QCD • Z/ge+e-with e+e- misidentified as photons e.g. • QCD processesthat in the final state contain : 1. two photons 2. a photonanda hadronic jet misidentified as photon 3. two hadronic jets misidentified as photons e.g. Alex Melnitchouk APS-DPF conference - 2003

  8. Selection of gg Candidate Events • Trigger: di-EM* high pT trigger • Offline:(on both objects) • Kinematic cuts: pT > 25GeV • Acceptance cuts: Central or End Cap Calorimeter up to ||=2.4 • Photon ID: -shower shape consistent with EM* shape (EMfraction, Isolation, H-matrix 2) • - track veto • *EM = Electromagnetic Object (Photon or Electron) Alex Melnitchouk APS-DPF conference - 2003

  9. Invariant Mass and  from Data and Signal MC DØ RunII Preliminary DØ RunII Preliminary Events / 3GeV Central Calorimeter Detectors differ slightly in mass resolution and ID performance Hadronic End-Cap Calori- meter EM look separately at : – both g’s central – both g’s in end-cap – central, end-cap EM Hadronic Alex Melnitchouk APS-DPF conference - 2003

  10. DØ RunII Preliminary Events / 8 GeV DØ RunII Preliminary Events / 8 GeV Events / 8 GeV DØ RunII Preliminary gg Mass Distributions and Event Counts (Ldt=52pb-1) Alex Melnitchouk APS-DPF conference - 2003

  11. Higgs Cross-Section Limits (Assuming B(gg)=1) • Make a counting experiment in the optimized mass resolution dependent sliding mass window G.Landsberg , K.MatchevPhys. Rev. D62, 035004 (2000) DØ RunII Preliminary  Ldt=52pb-1 Experimental Upper Limit @ 95% CL • ApproachingB(gg)-Mh region of interest • To increase sensitivity : need analysis optimization and better understanding of photon misidentification rate Alex Melnitchouk APS-DPF conference - 2003

  12. Conclusions • A Search for the Topcolor Higgs was performed in the inclusive h gg + X channel with 52pb-1 of DØ data collected between September 2002 and January 2003 • Measured gg spectrum is in agreement with the Standard Model background predictions • First attempt to set Cross-Section Limits with DØ RunII gg data has been made • Work on improving sensitivity is in progress • Another search for the Higgs with suppressed fermion couplings at DØ (HWWtalk by Peter Tamburelo tomorrow) • More data is being accumulated • Stay tuned for new results Alex Melnitchouk APS-DPF conference - 2003