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

Chapter 4: Data Collection and Event Reconstruction 128
of position, direction and momentum. Note that a pattern typically contains hits in
multiple MDT stations.
Segment building
In this step, hits in the patterns from the pattern recognition step are used to
form short, straight track segments in each MDT chamber. As shown in Figure 4.10
(top left), for a pair of hits in the two outermost layers of a multilayer, the four
possible tangent lines connecting the drift circles are drawn. A tangent line is kept if
its direction is within 0.2 radian of the direction of the corresponding pattern, and if
there are at least three hits within 1.5 mm of the tangent line. A χ 2 fit is performed
using distances to the drift circles. If a segment has a χ 2 per degree of freedom larger
than 10, the hit which has the largest contribution to the χ 2 is rejected, and the
segment refitted. This sequence is repeated until the χ 2 per degree of freedom is
less than 10, or there are less than three hits left. At this point we have a segment
candidate.
Trigger chamber hits are now added to the segment. In the barrel, hits in RPC
chambers located on the middle and outer MDT stations are added to segments in
the corresponding MDT chambers without any refit. In the endcap, TGC hits are
similarly added to segments in the inner and middle MDT stations.
A fraction of the segment candidates at this stage can be ambiguous owing to hit
sharing. To resolve ambiguities, segments are scored using a set of quality criteria in
the following order of priority:
1. number of hits in precision chambers
2. number of missed hits (holes) in precision chambers