Hi,
I have been thinking about this k factor and just realised that we CAN
NOT directly measure the size of the cell and adjust the k factor
accordingly. The time to position algorithm converts the drift time as
a function of wire, superlayer, B, and track angle to a position in the
plane of that sense wire but that position is not limited to be within
the cell as obviously an angled track can cross the sense wire plane
outside of the cell and be tangential to the isochrone inside the cell.
Thus the electron drift is inside the cell but the distance from the
sense wire, within the plane of the sense wire, is outside the cell.
Thus the distance from the sense wire which we use in tracking is not
limited to the cell size. I expect this could explain the range of k
factors Wang found changing from large angles relative to the line
normal to the sense plane where this affect is greatest to back angles
where the tracks where the tracks are closer to the normal vector.
The k factor is still an important adjustment to the drift velocity
but to determine it we must specifically select events which cross the
cell boundary and line up the hits in one cell with the hits in the
neighbouring cell. This is not easy as at most there are two hits in
one cell and one in the neighbouring cell.
Hope this is clear to everyone. Otherwise we should meet and I can
draw a few pictures to explain the problem.
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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