Hi,
After Chris' s presentation yesterday I looked at how nsed at least
calculates the track position and it assumes the the track crosses the
cell at 0 degrees ie perpendicular to the chamber. I don't know where
to look to say whether or not the reconstruction does this properly or
not. The so called default angle which is in the wire calibration file
varies from -53 degree for cells at 20 degrees to 8 degrees for the
cells at 80 degrees. So nsed at least does not use the default angle
but rather fixes the track angle to be 0.
Now it has been stated that using the track angle results in worse
fits. My question here is then at what point is the proper track angle
used? If the stubs forming the track are selected based on the results
using 0 degrees and then the same stubs are recalculated using the
track angle this may indeed result in poorer resolution. The proper
procedure is to use the default angle as in the wire calibration file,
determine the stubs from those positions which could result in
different combinations of hits being chosen, and then refine it with
the proper track angle once the track is found. Then the change should
not be so great and possibly improves things. Going from 0 degrees to
the actual track angle is a big change (eg a change of -50 degrees in
the forward direction).
The time to distance curve Chris showed yesterday showed the results
of his improved fit and compared it with the "GARFIELD" result.
Chris's result rose quickly from zero and then went linear about 2 mm
above the GARFIELD result which rose less steeply from zero and then
went linear. This is the behaviour using 0 degree track angle in a
magnetic field. With higher track angles the rise is steeper. I
suspect that if the GARFIELD result had been given the correct track
angle the discrepancy with Chris;s result would not be as great.
Chris: If you tell me which chamber, cell, layer, etc. you used in
that plot I will calculate the correct "default" time to position
curve.
Cheers,
Douglas
26-415 M.I.T.
Tel: +1 (617) 258-7199
77 Massachusetts Avenue Fax: +1 (617)
258-5440
Cambridge, MA 02139, USA E-mail:
hasell@mit.edu
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