The measurement of the W pair production cross-section at the Tevatron
provides an important
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.
could result in an enhanced rate of
W pair production.
Standard Model WW production is also an important background
heavy Higgs signal :
where the ggH vertex is via a quark loop, and is also a background to
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
expected background of 37.8± 0.9
consistent with the Next-to-Leading-Order
calculation of the Standard Model WW cross-section, 12.4 ± 0.8
(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
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
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.
The addition event requirements are:
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 (giftexeps):