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

Chapter 4: Data Collection and Event Reconstruction 100
Outside-in track reconstruction
Since inside-out reconstruction requires a minimum number of silicon hits to form
track seeds, it is inefficient for tracks that originate in the tracker after the Pixel layers.
In particular, it can miss tracks from Ks decays and photon conversions. Such tracks
can be recovered using an outside-in tracking algorithm which starts in the TRT. In
the first stage, TRT track segments are formed using a histogramming technique [17].
Only those TRT hits that have not been assigned to tracks in the inside-out recon-
struction are used. Rough estimates of the transverse track parameters, namely φ, r
in the barrel and φ, Z in the endcap, are associated with the segments.
In the second stage, the segments are extended into the silicon detectors, and
space-point seeds are formed in a narrow r − φ road. Space-points that have already
been associated with a track during the inside-out reconstruction are not used in the
seed search. Only the three outermost layers of SCT are used, a minimum of two
space-points being required within the road. The space-points yield improved track
parameters, which are then used to redefine the road in the silicon detectors. As in
the case of inside-out tracking, a combinatorial Kalman fitter/smoother algorithm is
used to add clusters and produce silicon track extensions, which are extended into the
TRT as before to form a global track. After ambiguity-resolving and track refitting,
the final tracks are stored in a dedicated track collection.
TRT-only track reconstruction
Following outside-in tracking, the remaining TRT segments that have not been
assigned silicon extensions form the basis of TRT-only tracks. Tracks parameters