Measurement of the Z boson cross-section in - Harvard University ...

Chapter 7: Background Estimation 219
isolation cuts to remove most of these events. The W → µν cross-section is known to
NNLO accuracy in QCD, but jets cannot be modeled with any accuracy in a Monte
Carlo program. Therefore, as with the QCD multijet background, the W → µν back-
ground is best extracted from data. However, again in view of the small size of our
data sample, we use Monte Carlo to estimate this background.
To all electroweak backgrounds, we assign a 3% systematic owing to PDF uncer-
tainty. In addition, we assign a further 5% theoretical cross-section uncertainty to
the W → µν and Z → ττ backgrounds, and a 6% theoretical uncertainty to the t¯t
background.
7.3 Cosmic ray background
High-pT muons from cosmic rays can potentially mimic the Z signal. Our collision
event criteria, in particular the cut on primary vertex z0 and the requirement that
the event have at least one primary vertex with at least three tracks associated with
it, largely reduce this background. However, cosmic muons that traverse the detector
near the beam spot and within the time window for a bunch crossing can look like
a Z → µµ event by appearing to be two oppositely charged energetic muons from
the interaction region. Since most bunch crossings lead to minimum-bias events, for
a cosmic muon to fake a Z decay it must coincide with a minimum-bias event, the
event must pass our muon trigger, and the muon track must be reconstructed as two
combined muons which pass the Z selection criteria.
In this section, we estimate an upper limit to the in-time cosmic contamination