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

Chapter 4: Data Collection and Event Reconstruction 130
3. number of hits in trigger chambers
If two segment candidates are ambiguous, the lower-scoring one is rejected.
In the segment combination stage, a second pattern recognition step is performed
using segments. First, the segments are ranked according to a second set of criteria
including isolation in the chamber and the crude momentum estimation from the
initial pattern recognition [88]. Compatibility between pairs of segments is then
estimated starting with the highest-ranked segment. Two segments are said to be
compatible if they come from the same initial pattern and have consistent position
and direction. If they are compatible and do not already belong to a track candidate,
they form a track candidate. If one segment belongs to a track candidate, the other
segment is assigned to the same candidate.
Once this process is complete, each track candidate is fitted using χ 2 minimization
with respect to the segments on it, and with the constraint that a track must pass
close to the interaction point. Material effects are not taken into account at this
stage. If two track candidates share 50% or more segments, they are merged.
Track finding
The first stage of track finding is track building, which is similar to the segment
combination described above except that material effects are taken into account and
more stringent quality criteria are applied. A track is built up segment by segment
starting from the outer MDT station to the inner stations 18 .
For a given track candidate, the segment in the outer MDT station with the high-
18 Since the outer muon stations suffer from less combinatorial background than the inner ones,
starting with segments in the outer stations results in a smaller number of possible combinations
and therefore faster track building