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

Chapter 4: Data Collection and Event Reconstruction 132
detailed description of both active and inactive material.
The next stage is track cleaning, which is itself a two-step procedure. In the first
step, an entire MDT station is removed from a track if the average pull of the hits
on track in that station is larger than 3.5, and if this pull does not result from any
single hit. In the second step, the drift circle belonging to each MDT hit is tested.
A track could have traversed on either side of a drift circle; in some cases the wrong
side may have been chosen during segment-making (see Figure 4.10). For each hit, it
is assumed that the track traversed on the other side of the drift circle from the one
originally chosen, and the pull due to that hit recomputed. If the pull is now lower
than the previous value, the new direction is kept and the track refitted.
For various reasons, a muon can traverse a layer in a chamber without registering
a hit (see Section 4.2.4 below). An attempt is made to add some of these hits back
to the track. In all chambers that have at least one hit on a track, layers without
hits are searched for. If such a layer has a channel that registered a signal, a hit in
that layer is added to the track provided that it has a pull smaller than a specified
value. Otherwise, the layer is flagged as a hole on that track. A search for missed
segments is then performed by checking that the track has segments in all three muon
stations. If one station is missing, a segment search is done in that station using the
parameters of the fitted track as a seed. If a segment is found, the hit/hole search as
described above is performed on it, following which the segment is either discarded
or added to the track.
At this stage, tracks can share hits. To assign each hit unambiguously to a track,
the tracks are ranked according to quality criteria using the number of hits and the