Measurement of the W+W- Production Cross Section in 825 pb-1 of ppbar Collisions at sqrt(s)=1.96 TeV using Dilepton Events

Shih-Chieh Hsu, E. Lipeles, K. McCarthy, M. Neubauer, F. Wurthwein
This is a preliminary analysis that is similar to the published CDF Run II result using 200 pb-1 published article.

Physics Background

The measurement of the W pair production cross-section at the Tevatron provides an important test
of the Standard Model. The leading order t-channel (~90%) and s-channel (~10%) diagrams are :

The s-channel diagrams give us experimental acess to WW-gamma or WWZ triple gauge couplings.
Anomalous couplings could result in an enhanced rate of W pair production.

Standard Model WW production is also an important background for a heavy Higgs signal :

where the ggH vertex is via a quark loop, and is also a background to t-tbar production,
and searches for new heavy gauge bosons.

Summary of Results

We present a measurement of the WW cross-section using 825 pb-1 of data. With a sample of 95 events
and an expected background of 37.8± 0.9 ± 4.7, we find

,
consistent with the Next-to-Leading-Order calculation of the Standard Model WW cross-section, 12.4 ± 0.8 pb.

(Note that the luminosity-dependent uncertainty is not the usual 6% because some background estimates scale with luminosity.)

Summary of Method

The WW cross-section is measured in the
two leptons and missing transverse energy (MET)
final state, where it is the primary contributor.  The other contributions to this final state are
• Drell-Yan,
• ttbar,
• WZ and ZZ,
• Wgamma where the gamma fakes an electron, and
• W+jets where a jet fakes a lepton.

We use the following categories of electron and muon for the two leptons
• electrons can be:
• Central(CEM): Et > 20 GeV and |eta|<1.1
• Forward(PHX): Et > 20 GeV and 1.2<|eta|<2.0
• muons can be:
• Triggerable(CMX and CMUP): Pt > 20 GeV, |eta|<1.0, fiducial in muon chambers
• Non-Triggerable(CMIO): Pt > 20 GeV, |eta|<1.2, does not have a muon stub attached
We use all combinations of these categories except the the CMIO-CMIO which is not triggerable.

• Two high-Pt leptons, both isolated
• The two lepton come from the same vertex ( DeltaZ < 4.0 cm)
• If both leptons are same flavour and 76<Mll<106 require MET/sqrt(sum transverse calorimeter) > 3.0
• Missing Et (MET) > 25 GeV
• Delta-phi (MET, nearest lepton) > 0.3 if MET < 50 GeV
• No jets with Et > 15 GeV (corrected), and |eta| < 2.5
• Both leptons of opposite charge
Summary of Backgrounds, Expected Signal, and Data

The Drell-Yan, ttbar, WZ, ZZ, and Wgamma backgrounds are modeled with a Monte Carlo simulation.
The W+jets background was modeled with a fake rate proceedure, extractring fake rates jet triggered
data and applying them to the lepton+jets sample. The following table shows the full list of backgrounds,
the Standard Model WW expectation and the data yield in each channel (gif tex eps):

Smaller Summary Table (gif jpg tex eps):

Kinematic Distributions (click on images for eps versions!)

Event Displays

Last Updated March 27, 2006 --Elliot Lipeles