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Alex Melnitchouk APS- DPF conference - 2003 1 Search for the Higgs Boson in the h Channel APS-DPF 2003 Philadelphia Alex Melnitchou Brown University on behalf of the DØ collaboration
Alex Melnitchouk APS-DPF conference - 2003 1
Search for the Higgs Boson in the h
Channel APS-DPF 2003 Philadelphia
Alex Melnitchouk Brown University
on behalf of the DØ collaboration
Alex Melnitchouk APS-DPF conference - 2003 2
Leading SM Higgs Production Processes at Tevatron
80 100 120 140 160
0.01
0.1
1.0
10.0
Higgs Mass, GeV
Cross-Section, pb
s = 2 TeV
gluon fusion : cross-section ~ m2 the top-quark loop is dominant
(Z*)
(Z)W/Z associated
W/Z fusion
Alex Melnitchouk APS-DPF conference - 2003 3
h decay in the Standard Model
• no tree-level H coupling (Higgs is neutral)
• diphoton decays happen via W or top-quark loop:
H 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 decay rate of the Higgs
Alex Melnitchouk APS-DPF conference - 2003 4
h 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 rate is also predicted by some models that employ Large Extra Dimensions L. Hall, C.Kolda, Phys. Lett. B459, 213 (1999)
• in addition 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 decays
h Branching Fraction
Higgs Mass, GeV
Standard Model
no couplings to fermions (Fermiophobic Higgs)
no couplings to down-type fermions
in general we should be prepared for any h branching fraction ( up to 1.0 ) due to new physics
S.Mrenna, J.Wells, Phys. Rev. D63, 015006 (2001)
no couplings to top,bottom quarks
Alex Melnitchouk APS-DPF conference - 2003 6
h Search Strategy Focus on 2 Scenarios
1 Fermiophobic Higgs (does not couple to fermions)• Production: W/Z associated + W/Z fusion • Main signature with diphotons : + 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)
2 Topcolor Higgs (of all fermions couples only to top)• Production: all three leading processes • Main signature with diphotons :
3 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() Focus : for the Topcolor Higgs assuming SM couplings
Alex Melnitchouk APS-DPF conference - 2003 7
Major Backgrounds : Drell-Yan and QCD
• Z/e+e- with
e+e- misidentified as photons
1. two photons
e.g.
3. two hadronic jets misidentified as photons
2. a photon and a hadronic jet misidentified as photon
• QCD processes that in the final state contain :
e.g.
Alex Melnitchouk APS-DPF conference - 2003 8
Selection of 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
Central Calorimeter
End-Cap Calori- meter
EM
Had
ron
ic
Eve
nts
/ 3G
eV
DØ RunII Preliminary DØ RunII Preliminary
EM Hadronic
look separately at :
– both ’s central
– both ’s in end-cap – central, end-cap
Detectors differ slightly in mass resolution and ID performance
Alex Melnitchouk APS-DPF conference - 2003 10
Mass Distributions and Event Counts
(Ldt=52pb-1)
Eve
nts
/ 8
GeV
Eve
nts
/ 8
GeV
Eve
nts
/ 8
GeV
DØ RunII Preliminary
DØ RunII Preliminary
DØ RunII Preliminary
Event Count (Central,Central)
QCD background : 411 48
Drel-Yan background : 107 24
Total background : 519 53
Data : 577
Event Count (Central,End-Cap)
QCD background : 507 34
Drel-Yan background : 182 24
Total background : 689 42
Data : 707
Event Count (End-Cap,End-Cap)
QCD background : 232 16
Drel-Yan background : 42 14
Total background : 275 21
Data : 322
Alex Melnitchouk APS-DPF conference - 2003 11
Higgs Cross-Section Limits (Assuming B(
• Approaching B(h region of interest
• To increase sensitivity : need analysis optimization and better understanding of photon misidentification rate
• Make a counting experiment in the optimized mass resolution dependent sliding mass window
Experimental Upper Limit @ 95% CL
Ldt=52pb-1
DØ RunII Preliminary
G.Landsberg , K.Matchev Phys. Rev. D62, 035004 (2000)
Alex Melnitchouk APS-DPF conference - 2003 12
Conclusions
• A Search for the Topcolor Higgs was performed in the inclusive h X channel with 52pb-1 of DØ data collected between September 2002 and January 2003
• Measured spectrum is in agreement with the Standard Model background predictions
• First attempt to set Cross-Section Limits with DØ RunII data has been made
• Work on improving sensitivity is in progress • Another search for the Higgs
with suppressed fermion couplings at DØ (HWWtalk by Peter Tamburelo tomorrow)
• More data is being accumulated• Stay tuned for new results
