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

Chapter 6: Event Selection 165
Figure 6.1: Background versus signal efficiency for three different cone sizes used to
define the relative track isolation. Monte Carlo samples are used for both background
and signal.
6.1.1 Properties of the preselected sample
In this section, we present some properties of the data events, Monte Carlo signal
and Monte Carlo background events after the preselection criteria have been applied.
The backgrounds we consider include t¯t, QCD (b ¯ b and c¯c), Z → ττ and W → µν.
These channels will be discussed in some detail in Chapter 7.
Hit distributions
We show below the distribution of hits on muon tracks in the various subsystems
of the muon spectrometer and the inner detector. Figure 6.2 shows the number of
MDT hits per track. The distribution peaks at 20 hits, which is expected for tracks
that traverse all layers of all three MDT stations (Section 2.2.4). However, we see a
significant number of tracks with 12 hits, i.e. , tracks that go through two stations.
Since at least 2 segments are required for a track to be reconstructed, and each