Search for a Standard-Model Higgs Boson
in WW Dilepton Decay
Channels with 360/pb Run II Data at CDF

Authors : Shanhuei Chuang, Mircea Coca, Mark Kruse : for the CDF Collaboration


Abstract

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). 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.



Introduction

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 < MH < 800 GeV), is in addition an indicator of new physics scale.

The best SM Higgs production mode cross-sections for the Higgs productions at Run II TeVatron as a function of the Higgs massIn 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 SM Higgs boson branching fractionsWhen 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 < MH < 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 -> WW -> lvlv (leptons + neutrinos), is shown below.

Feynman diagram for the SM gg -> H -> WW* -> lvlv process
an aeroview of Fermilab.


Apparatus

an isometric view of the CDF II detector 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.



Event Selection

We identify signal events with following requirements:

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 to extract the 95% confidence level (CL) production limit as a function of Higgs mass.

electron fake rate fake rate for electrons gif eps and for muons gif eps
jet Et spectrum of 1-jet events jet Et spectrum of 1-jet gif eps and 2-jet gif eps events
leading lepton pt spectrum at initial level leading lepton pt spectrum at initial level gif eps and final level in log scale gif eps or linear gif eps
subleading lepton pt spectrum at initial level subleading lepton pt spectrum at initial level gif eps and final level in log scale gif eps or linear gif eps
dilepton invariant mass distribution at initial level dilepton invariant mass distribution at initial level gif eps; N-1 for 130 gif eps, 150 gif eps, 160 gif eps, 180 gif eps GeV
missing Et distribution at initial level missing Et distribution at initial level gif eps; N-1 for 130 gif eps, 150 gif eps, 160 gif eps, 180 gif eps GeV
sum of lepton pt + missing Et distribution at initial level sum of lepton pt + missing Et distribution at initial level gif eps; N-1 for 130 gif eps, 150 gif eps, 160 gif eps, 180 gif eps GeV
missing Et significance at initial level missing Et significance at initial level gif eps and final level in log scale gif eps or linear gif eps
Ht spectrum at initial level Ht spectrum at initial level gif eps and final level in log scale gif eps or linear gif eps
cluster mass at initial level cluster mass at initial level gif eps and final level in log scale gif eps or linear gif eps
transverse mass at initial level transverse mass at initial level gif eps and final level in log scale gif eps or linear gif eps


Results

gg->H->WW->lvlv signal acceptance gg->H->WW->lvlv signal acceptance gif eps
signal cut efficiencies cut efficiences on signal MH = 160 GeV
uncertainties on signal acceptance signal acceptance uncertainties and backgrounds background uncertainties

grand summary table per 5 masses
grand summary table for 110-150 GeV110-150   grand summary table for 160-200 GeV160-200
summary table for each mass
summary table for 110 GeV110   summary table for 120 GeV120   summary table for 130 GeV130   summary table for 140 GeV140   summary table for 150 GeV150  
summary table for 160 GeV160   summary table for 170 GeV170   summary table for 180 GeV180   summary table for 190 GeV190   summary table for 200 GeV200  


all the tables in a row (postcript)


dilepton azimuthal angle distribution at final level dilepton azimuthal angle distribution at final level dilepton azimuthal angle distribution at final level dilepton azimuthal angle distribution at final level dilepton azimuthal angle distribution at final level
dilepton azimuthal angle distribution at final level dilepton azimuthal angle distribution at final level dilepton azimuthal angle distribution at final level dilepton azimuthal angle distribution at final level dilepton azimuthal angle distribution at final level

dilepton azimuthal angle distribution at final level for 110-200 GeV, used to extract the gg -> h0 -> WW production cross-section limits


SM gg -> H -> WW production limits



summary table for the gg->H->WW->lvlv cross-section limits



combined SM Higgs search results from CDF          combined SM Higgs search results from CDF


Conference Note CDF7893
Minor Correction


last modified by Sunny on Sun Dec 25 12:12:12 CST 2005