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

Chapter 6: Event Selection 167
with multiple hits in the same layer.
We note that, in the peak region, the data is systematically shifted toward lower
number of hits compared to Monte Carlo. This is primarily due to misalignment
among the MDT stations that is not taken into account in the simulation. The
misalignment can result in some MDT hits not being assigned to a track, thus shifting
the hit spectrum toward lower values.
Figure 6.3 shows the number of RPC η and φ hits per track. Since there are 3
RPC stations, and each station has 2 readouts in η and 2 in φ, 6 hits are expected
in each coordinate for a track that traverses all three RPC stations. This is where
the peak occurs in each coordinate. Tracks with more than 6 hits in η or φ have
gone through the overlap region between adjacent sectors. In both coordinates, we
see good agreement between data and Monte Carlo. We note that the 0-hit bin is
due to tracks in the endcap which are not expected to traverse the RPCs.
Figure 6.4 shows the number of TGC η and φ hits per track. The expected number
of hits per track in each of the η and φ coordinates is determined by the arrangement
of doublet and triplet gas gap configurations, as described in [15] (see also Figures
1-20 and 4-30 in [15]). In Figure 6.4, we see that the data is significantly above the
Monte Carlo expectation in some bins. For the η coordinate, we see this discrepancy
in the 1-, 5- and 15-hit bins, while for the φ coordinate this is the case in several bins.
The discrepancy is explained by cross-talk between neighboring readout channels. In
Monte Carlo, the amount of cross-talk is determined by a parameter that must be
adjusted such that the Monte Carlo hit distribution agrees with that in data. This
adjustment has not been done in the Monte Carlo samples we use, so that some