Outside-In Tracking Description
"Outside-in" (OI) tracking is one component of the
SiPatternRecModule.
It takes COT tracks found by the
CT_TrackingModule and extrapolates them into
the silicon detectors, adding hits via a progressive fit. The algorithm is
very similar to that used in Run 1. As each layer of silicon is encountered
(going from the outside in), a road size is established based on the error
matrix of the track; currently, it is four standard deviations big. Hits
that are within the road are added to the track, and the track parameters
and error matrix are refit with this new information. A new track candidate
is generated for each hit in the road, and each of these new candidates are
then extrapolated to the next layer in, where the process is repeated.
As
the extrapolation proceeds, the track error matrix is inflated to reflect
the amount of scattering material encountered. At the end of this process,
there may be many track candidates associated with the original COT track.
The candidate that has hits in the largest number of silicon layers is chosen
as the winner; if more than one candidate has the same number of hits, the
chi squared of the fit in the silicon is used as a tie-breaker.
The OI tracking requires that COT tracking be performed first. The
SiPatternRecModule passes the COT tracks to the OI algorithm. The output
is a set of CdfTracks that are marked as outside-in tracks; these can then
be stored in the usual fashion for CdfTracks.
OI tracking is (currently) a default in SiPatternRecModule, and is
in fact the primary algorithm for silicon tracking. This can be changed
by the user in the talk-to for the module.
Ken Bloom, 23 July 1999.