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.
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):
,
