I previously estimated the location of the IFCE short in Run 17 (first appeared on 2017-04-13) via laser data to be somewhere near ring location 45.5. That initial estimation was never followed up by any thorough study. Now...

Matt Posik has noted during the Run 17 AuAu54 calibration of SC&GL that there seems to be an asymmetry. All distortion corrections appear to be applied properly, including the shorted ring at IFCE 45.5. But there seems to still be an issue with signed DCAs that shows z dependence, which indicates something amiss in the volume of the TPC, not at the endcap.

Below are a series of signed DCA vs Z_vtx [cm] plots with various shorted ring correction locations for IFCE that seem to point to a location perhaps near 10 rings in from the central membrane. For SC&GL, I used the result of Matt's calibration after 4 passes in which the east-west asymmetry was forced to be 1.0.
(cuts: |Z_vtx|<100 cm, |η|<0.05, plus other cuts using mode 51 of the track-by-track comparison code; processed 5k events from file st_physics_18166037_raw_2000021.daq)

No correction	ring 45.5
ring 20.5	ring 10.5	ring 1.5

Ring 1.5 appears to be too far. And ring 10.5 is pretty close, but maybe not quite far enough. So I will proceed with using ring 9.5.

It turns out that the Run 17 pp510 data did not show the need for this asymmetry because it was calibrated using only runs earlier than day 103 when the short began. This fortunately means that the incorrect ring position did not ruin the calibration.

Here is the impact on reconstructed pT of real tracks (passing reasonable quality cuts for the track-by-track comparison) in terms of delta(pT)/pT2 vs. vertex z (for narrow eta) and vs. eta (for narrow vertex z). The maximal impact is ~1.5% of 1.0 GeV tracks, and consequently half that (~0.75%) for 0.5 GeV tracks.



-Gene

_____________

Update:

- TOF: Daniel Brandenburg looked for any changes in calibration performance before/after day 103 and saw nothing.
- TPC dE/dx: Yuri looked similarly at time-dependence of calibration vs. day and saw nothing (perhaps remaining spread is greater than the impact)
- BeamLine: I saw nothing change at day 103.
- <sDCA> in calibration event.root: I looked and saw what might be a mild change at day 103: mean of <sDCA> for after day 103 (excluding RHICf) is 0.16 cm higher than for before day 103. This is inclusive of east and west, so local affects near the east side central membrane are likely washed out.

My conclusion is that all of the above were likely dominated by low p_T tracks where the impacts were rather small. Thus, the calibrations that were done despite the incorrect distortion correction were not significantly impacted. The calibrations need not be re-done!

However, high p_T tracking should be impacted. A 40 GeV/c track may be incorrect by as much as 1.5% * 40 = 60%!!! (Of course, it's really 1/p_T that is off by 60%, so 40 GeV/c track becomes either 25 GeV/c or 100 GeV/c depending on its charge sign). Thus, I have decided to go forward with using the new shorted ring position (9.5) for the st_W production that is beginning: its entryTime in the database is 2017-09-14, and the production is using DbV20170915.

-Gene