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 :

wwu
wws

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 :

wwu

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

                      $\displaystyle \sigma = 13.6 \pm 2.3 (stat) \pm 1.6 (sys) \pm 1.2 (lum) pb$,
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

We use the following categories of electron and muon for the two leptons
We use all combinations of these categories except the the CMIO-CMIO which is not triggerable.

The addition event requirements are:
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