I redid the analysis of the offsets of the middle drift chamber in
each sector based on matching the momentum and angle for elastic
events. (see http://blast.lns.mit.edu/PRIVATE_RESULTS/USEFUL/
ANALYSIS_MEETINGS/meeting_050830/wc_ffr.ppt )
Explanation:
All three figures are plots of the same WC offsets, as a function
of different variables: theta,phi,z
In each plot, the y-axis is the fitted shift of the middle WC in cm
(3x Doug's value).
The top row in each figure is the offsets calculated from electron
tracks, and bottom row protons; each for the left and right sectors.
Observations:
Obviously, the 1.2 mm shift in the right sector is gone. We are
left with a strong dip (1.5 mm) at very forward angles, and then
increasing gradually from -.5 to +.5 mm as a function of theta. But
this is fairly uniform between the two sectors, and there is not such
strong agreement between from electrons and protons as before. I see
a little of the structure matched by the electrons and protons in the
right sector on the order of 0.2 mm but one would need to match x_wc
(theta_e) and x_wc(theta_p) to be sure it was due to geometry.
Also, the dip around phi=0 in the right sector (electrons) has
disappeared due to corrections to the magnetic field. The sharp 2 mm
drop-off at phi<-10 deg in the left sector has also softened (new
field map?), and there is only a slight roll-off at the edges.
The z-dependence also looks good, with the exception of z~-20,
which may be an artifact or poor reconstruction at small theta.
Conclusion:
The inter-WC geometry is most likely OK now. Since there is
fairly good agreement between the two sectors, the residual offsets
we now see could be due to other factors such as radiation, or
overall position/rotation of the wire chambers in each sector as a
whole. I will try an exercise to fit for these parameters in the
future.
--Chris
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