Search for the Higgs Boson in the hgg Channel

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Search for the Higgs Boson in the h?gg Channel
APS-DPF 2003 Philadelphia
Alex Melnitchouk Brown University on behalf of
the DØ collaboration
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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

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h?gg decay 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

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h?gg decays 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 gg rate is also predicted by some models
  that employ Large Extra Dimensions
  L. 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

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Examples of Enhancement of h?gg decays
h?gg Branching 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
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h?gg Search 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 CL hep- 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
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Major Backgrounds Drell-Yan and
QCD

• Z/g??ee- with ee-
  misidentified as
  photons

e.g.

• QCD processes that in the final state contain

1. two photons
2. a photon and a hadronic jet misidentified
as photon

• 3. two hadronic jets misidentified as photons
  

e.g.
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Selection
of gg Candidate Events

• Trigger
  di-EM high pT
  trigger
• Offline (on both objects)
• Kinematic cuts pT gt 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)

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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 gs central both gs in
end-cap central, end-cap
EM
Hadronic
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gg Mass Distributions and
Event Counts (?Ldt52pb-1)
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Higgs Cross-Section Limits (Assuming B(gg)1)

• Make a counting experiment in the optimized
  mass resolution dependent sliding mass window

G.Landsberg , K.Matchev Phys. Rev. D62, 035004
(2000)
DØ RunII Preliminary
?
Ldt52pb-1
Experimental Upper Limit _at_ 95 CL

• Approaching B(gg)-Mh region of interest
• To increase sensitivity need analysis
  optimization and better understanding of photon
  misidentification rate

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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?WW
  talk by Peter Tamburelo tomorrow)
• More data is being accumulated
• Stay tuned for new results