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

Chapter 6: Event Selection 169
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Data 2010 ( s=7
TeV)
Z → µ µ
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W → µ ν
Z → τ
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TGC eta hits/track
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dt = 331 nb
Data 2010 (
Z → µ µ
bb
cc
W → µ ν
Z → τ
tt
s=7
TeV)
0 5 10 15 20 25
TGC phi hits/track
Figure 6.4: TGC η (left) and φ (right) hits per muon track for signal Monte Carlo,
background Monte Carlo and data after event preselection. The total Monte Carlo
event number has been normalized to the number of events in data.
Although the data agrees with Monte Carlo expectation in most bins, we see that
it is significantly above expectation in the 0- and 2-hit bins. These tracks are most
likely TRT-only tracks that were back-extrapolated into the silicon, and have few or
no SCT hits [12]. We note that these tracks constitute ≈ 0.1% of the total sample.
A track is typically expected to traverse 36 TRT layers in the barrel, which is
where we see the peak in the hit distribution. The smaller peak at ≈ 20 hits is due to
tracks in the TRT endcaps. The 0-hit bin is due to tracks outside the TRT acceptance
of |η| < 2.0.
As with the MDT hit distributions, we see a migration toward lower bins in data
compared to Monte Carlo expectation in the TRT hit distribution. As before, we
understand this feature as a consequence of misalignment among the layers, so that
the algorithm fails to associate some hits with the track.