In the Standard Model, when the mass of Higgs boson is greater than 135 GeV the predominant mode of Higgs decay is to a pair of W bosons. Here we present a search for a single Higgs boson at CDF using this decay mode with both W bosons further decay leptonically (to electrons or muons only). We will extend the analysis to include the Higgs production in association with a W/Z boson later. The major background is inclusive WW production. However a distinguishing feature between the inclusive WW and the SM H --> WW process is the azimuthal angle between the two final-state leptons, the distribution of which we use to set a 95% CL limit on the gg --> H --> WW production (product of cross-section and branching ratio) as a function of Higgs mass.

Good reasons for the SM Higgs boson searches In the Standard Model (SM) and its supersymmetric (SUSY) extensions, the Higgs boson is crucial to our understanding of electroweak symmetry breaking dynamics and the mass generation of electroweak gauge bosons and fermions. The Higgs boson mass, which is yet a free parameter within quite loose bounds (114 < M_H < 800 GeV), is in addition an indicator of new physics scale.

The best SM Higgs production mode In proton-antiproton collision (what TeVatron provides) the dominant SM Higgs boson production mechanism is gluon-gluon fusion, gg --> h⁰, which cleanly produces a single neutral Higgs boson and not any by-products. The second dominant clean Higgs production mechanism, quark-quark fusion, is about two-order smaller. The figure to the right summarizes the SM Higgs production mechanisms available at TeVatron in cross-section vs Higgs mass at Run II energy.

The best SM Higgs decay channel When the mass of the Higgs boson is greater than 136 GeV the predominant mode of Higgs decay is to a pair of on/off-shell W bosons. Each of the W bosons can further decay into either lepton+neutrino or jets. From an experimenter's viewpoint, the case of both decay leptonically is the simplest for analysis because it is the most free of QCD contamination. The figure to the left summarizes various SM Higgs decay channels in branching ratio vs Higgs mass.

Combination of the bests In this analysis we search for the signals of a high-mass SM Higgs boson (135 < M_H < 185 GeV) that decays to a W pair and further to two leptons (electrons and muons only) plus neutrinoes. The Feynman diagram of the leading-order production mechanism and the chosen decay channel, (gluon-gluon) gg --> h⁰ --> W^(*) W^(*) --> lvlv (leptons + neutrinos), is shown below.

This analysis is carried out in Fermilab, where the TeVatron collides proton and antiproton beams at center-of-mass energy of 1.96 TeV in Run II.

This analysis is carried out using the data collected from May 02 to September 03 at CDF, one of the two collision points at which a delicate detector is constructed and well-maintained to get data. Shown to the right is an isometric view of the CDF II detector. Counting outwards from the beampipe central line, the detector is comprised of a silicon vertex detector (SVX II), a multiwire drift chamber (COT) for particle tracking, lead-scintillator electromagnetic calorimeters ({C/P}E{M/S}), iron-scintillator hadronic calorimeters ({C/W/P}HA) and drift-tube chambers and scintillators (C{M/S}{U/P/X}) for muon detection. Radiation hazards are dissolved with concrete shielding.

We identify signal events with following requirements:

• well-reconstructed two leptons, each with E_t > 20 GeV
• no jets with E_t > 15 GeV and |eta| < 2.5
• missing E_t > 25 GeV
• For missing E_t < 50 GeV, delta phi(missing Et, closest lepton or jet) > 20 degree
• not 76 < M_ee/uu < 106 GeV unless MetSig > 3
• leptons having opposite charge signs
• dilepton invariant mass M_ll < half Higgs mass. Our MC study shows the tendency of small HWW dilepton invariant mass, which is not a property of background SM processes. See to the right.

Further Higgs Discrimination The Higgs boson is predicted to have zero spin in the Standard Model, which would make a Higgs event different from background in many ways, for example, small angular separation and small invariant mass of the two final-state leptons. We select events with small dilepton invariant mass and fit the distribution of dilepton azimuthal angular separation (a quick peek) to extract the 95% CL (Confidence Level) production limit as a function of Higgs mass.

Results

signal and background expectations for the Higgs mass MH = 180 GeV (SM) source expectation WW 6.49 +/- 0.76 WZ 0.18 +/- 0.02 ZZ 0.06 +/- 0.01 Drell-Yan ee 0.87 +/- 0.44 DY mumu 0.43 +/- 0.19 DY tautau 0.03 +/- 0.01 ttbar 0.02 +/- 0.01 fakes 0.81 +/- 0.25 total bg 8.90 +/- 0.98 HWW 0.17 +/- 0.02 data 8

cluster mass MC = sqrt(pt ll2 + Mll2) + missing Et distribution of data, signal and background MCs, predicted to be large for the HWWlvlv signals because the final-state lepton pair and the neutrino pair tend to fly out at large angular separation.

dilepton azimuthal angular (delta_phi ll) distribution of data, signal and background MCs, predicted to be small for the HWWlvlv signals.

the delta_phi _ll distribution per 10 GeV Higgs mass increment for fitting and the extraction of signal production limits.

summary of all the important parameters in this analysis and the results
the total acceptance includes the branching ratio BR(W-->lv)²

Higgs mass (GeV)	140	150	160	170	180
cross-section(gg --> h0) (pb)	0.45	0.36	0.30	0.25	0.21
branching ratio(H --> WW)	0.48	0.68	0.90	0.97	0.94
integrated luminosity (pb-1)	184 +/- 11	184 +/- 11	184 +/- 11	184 +/- 11	184 +/- 11
total acceptance (%)	0.124 +/- 0.012	0.228 +/- 0.023	0.402 +/- 0.040	0.476 +/- 0.048	0.449 +/- 0.045
expected signal (event)	0.10 +/- 0.01	0.15 +/- 0.02	0.22 +/- 0.03	0.22 +/- 0.03	0.17 +/- 0.02
WW background (event)	3.51 +/- 0.41	3.82 +/- 0.45	4.45 +/- 0.52	5.38 +/- 0.63	6.49 +/- 0.76
other background (event)	0.68 +/- 0.16	0.90 +/- 0.24	1.34 +/- 0.35	1.91 +/- 0.47	2.40 +/- 0.55
candidate data (event)	2	2	3	7	8
95% CL limit - counting (pb)	18.4	9.8	6.2	8.2	8.8
expected limit - delta phi (pb)	18.1	9.8	6.0	7.4	8.0
95% CL limit - delta phi (pb)	17.8	9.4	5.6	5.6	6.4

our results in comparison with the prediction of Standard Model. We have excluded the region above our curve.

in combination with the results from WH --> lvbb Search at CDF. The right plot is the left one normalized to the SM prediction.

Reference

WW Cross-section Measurement at CDF

Presentations

CDF Collaboration Meeting, Fermilab, 04/29-04/30/04
PHENO 04, Madison WI, 04/26-04/27/04
APS APR04, Denver CO, 04/30-05/04/04
Fermilab Today 06/03/04
New Perspectives @ Fermilab, 2004
BEACH 04, Chicago IL, 06/27-07/03/04

CDF Public Note

CDF Internal Webpage
for the
SM Higgs Boson Search in the WW Dilepton Channels
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last modified by Sunny on Thu Aug 05 17:09:01 CDT 2004