Update on 2006 issues
FINDINGS:
BeamLine position does move with SpaceCharge(+GridLeak) correction. Here I plot the BeamLine constraint as a function of turning on the SpaceCharge correction 10% at a time:
The cause appears to be the Minuit Vertex Finder exhibiting something like a (x,y) = (0,0) bias which gets worse when the distortion corrections are not proper. To demonstrate this, I plot the beamline position as reconstructed using SpaceCharge,GridLeak,ShortedRing-corrected data (left) and uncorrected data (right). The center point in each plot is the "as-is" reconstructed beamline x and y intercepts. I shift all the hits in the TPC by +/-0,1,2 mm in x and y and re-calibrated the BeamLine for each shift. The blue dots represent those shifts given to the TPC hits, while the arrowheads point at the value that comes from the calibration for the respective shifts. Some bias appears to remain even with the calibrations in place, but the bias is clearly stronger for uncorrected data.
In this case, the uncorrected data is actually using calibrations as they were in June when the BeamLine calibration was originally done: no SpaceCharge, and only a partial Shorted Ring correction.
A few more things can be learned from the data by excluding sections of the TPC. In these plots, I show sector positions as they are in the TPC, divided into 12 supersectors with inner (padrows 1-12) and outer (padrows 14-45) subsectors. Each bin then represents the y-intercept of the beamline when I EXCLUDE the vertices which have any tracks passing through that part of the TPC. I include in text below the plot the y-intercepts for the minimum and maximum shown in the plots, and for the FULL TPC (not excluding any vertices). I can exclude some sectors simultaneously too. EAST IS ON TOP, WEST IS ON BOTTOM (unless I tied east and west exclusion together, so I only show one plot). The disc in the center of each plot represents the FULL TPC position for comparison.
First, using the corrections as they were in June, one can clearly see the effect caused by uncorrected SpaceCharge:
| Uncorrected | July corrections | August corrections |  |
 |
 |
Full TPC y = -0.060040 +/- 0.009377 Minimum y = -0.179608 Maximum y = 0.056453 |
Full TPC y = -0.266030 +/- 0.004800 Minimum y = -0.276851 Maximum y = -0.245626 |
Full TPC y = -0.259742 +/- 0.002232 Minimum y = -0.263649 Maximum y = -0.241834 |
|---|
In FullField data like this, SpaceCharge makes tracks rotate clockwise when viewed along the positive axis. So SpaceCharge-distorted tracks at positive x tend to want a higher vertex position, while ones at negative x want a lower vertex position. Therefore, excluding vertices which use tracks at positive x from uncorrected data will effectively lower the average vertex position, and vice versa for negative x. The uncorrected data is dominated by SpaceCharge, so both halves of the TPC look pretty much the same.
With Shorted Ring and SpaceCharge corrections as they were in July (SpaceCharge calibration had been done without the final Shorted Ring corrections), one can see that we were probably slightly over-correcting for this data, but not by much as the minimum and maximum were both within ~300 microns of using the whole TPC.
With the SpaceCharge+GridLeak re-calibrated using the final Shorted Ring corrections in place, the fully corrected plot now appears as in the August-corrections column above. The max-min diff has gone from 310 microns to 220 microns! And the west tpc (bottom) looks essentially flat: there is no distortion apparent in the beamline from the west TPC! The east still shows some remnant distortion. This could be due to either more SpaceCharge in the east than west, or due to not-quite-complete corrections of the shorted ring in the east TPC. Either explanation is viable.
These graphs also demonstrate that there is unlikely to be much contribution from misalignment of TPC (sub)sectors.
We can remove azimuthally symmetric distortions like our standard SpaceCharge+GridLeak and the Shorted Ring by tying together (excluding at the same time) opposing (in phi) subsectors of the TPC:
Full TPC y = -0.259742 +/- 0.002232
Minimum y = -0.263744
Maximum y = -0.243980
Here I think it is of interest to notice that the major cause for altered beamline in the east TPC is from the outer subsectors at 3 and 9 o'clock. Shorted ring distortions are primarily an inner sector effect and should not show up in this plot anyhow. This means that it is more likely to be due to additional ionization in the east TPC over the west.
We can also analyze the horizontal determination of the beamline.
Full TPC x = -0.009712 +/- 0.002242
Minimum x = -0.018511
Maximum x = 0.000031
Again, more distortions in the east TPC. Overall max-min diff here is only 185 microns! It's curious that this is not symetric about 6 and 12 o'clock. Tying opposing subsectors shows something else:
Full TPC x = -0.009712 +/- 0.002242
Minimum x = -0.015330
Maximum x = -0.002258
Both the east and west show something at 5 and/or 10 o'clock (and a bit at 4/10) contributing (minutely - were talking on the order of below 100 microns) to the horizontal position of the beam.