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

Chapter 6: Event Selection 161
beam halo and beam-gas interactions.
• Then we require that the events pass the L1 MU6 trigger. A this stage, the
data sample contains 5464740 events.
• Events are then required to have at least one reconstructed primary vertex
which has at least three inner detector tracks associated with it. In addition,
we require that the primary vertex be located within 15 cm from the nominal
interaction point along the beam pipe. This requirement takes the finite size
of the beam spot into account, at the same time reducing contamination from
cosmic events. 5103785 events pass the vertex cut.
The ‘high-pT muon event’ selection eliminates low-quality, low-pT and possibly
fake muons. We require that the events have at least one combined muon, recon-
structed in both the inner detector and the muon spectrometer, having pT greater
than 15 GeV and within |η| < 2.4. The later cut is motivated by the fact that the
muon trigger coverage ends at |η| =2.4 2.2.4.
A detailed study of high-pT muons in collision data [61] showed that requiring
a muon spectrometer track extrapolated to the primary vertex to have pT greater
than 10 GeV largely rejects muons from pion/kaon decays-in-flight as well as poorly
matched tracks between the inner detector and the muon spectrometer. Additionally,
requiring the difference in pT between the spectrometer and inner detector tracks,
|p MS
T −p ID
T |, to be less than 15 GeV significantly reduces fake muons while losing very
little of the signal. We therefore impose these two criteria on the muon track. 22075
events are left in the data sample at this stage.