This is the evidence I have seen for a distortion due to the outer sector 21 gating grid apparently not closing. This is from runs taken on day 349 before the anode high voltage for outer sector 21 was turned down the next day to kill ion generation at the endcap. The data is from FastOffline with an initial, uncalibrated correction for SpaceCharge & GridLeak based on prior years' data. The fact that it is uncalibrated likely explains the negative baseline offset of a little over 1 mm for all tracks.

Here I break the tracks into 4 subsets showing mean signed DCA (<sDCA>) vs. p_T and phi for east/west and -/+ tracks. The first set of 4 plots is a "surf1" profile plot, while the second set is a "cont4" profile. West side tracks show no notable distortion, while both + and - tracks on the east side show a sizable distortion centered around phi=0, which is the location of sector 21.




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I next looked for any luminosity dependence I could find. There was only a small amount of luminosity variation over this data of roughly ~10%. Here I show the zdcw scaler [Hz] vs. time [sec], and then a look at the "surf1" profile plot of east + tracks of <sDCA> vs. zdcw rate and phi with the vertical axis zoomed in to enlarge the small dependence on zdcw. It does look like about ~10% reduction in the distortion along with the ~10% reduction in luminosity.

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Update [2021-12-20]:

I re-made the plots using data from day 354 (runs 22354004-22354024), which has ZDC west rates of approximately an order of magnitude larger than the day 349 data used above. This data has TPC sector 21 outer anode HV turned down to something like 800 V (where nominal is 1390 V) and the RDOs turned off so that there are no noise hits. The baseline is shifted with respect to the day 349 data, which I suspect still relates to the fact that SpaceCharge is not properly calibrated, so the whole TPC has a remaining uncorrected distortion. On top of that, however, there does still appear to be a feature centered around sector 21 at phi=0 on the east side. One possibility is that the tracks I use from that region of space are a biased sample because of the improper SpaceCharge correction. Another possibility is that there is still some ion backflow at this reduced anode HV setting.


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And using data from days 351-354, I'm able to look at some luminosity (ZDC west singles rate) dependence, as day 351 was typically ~50 kHz, day 352 was ~100 kHz, day 353 was mostly above 200 kHz, and day 354 was mostly below 200 kHz. So here's the 4 track groups vs. zdcw still showing some effect around sector 21:



There does appear to be some zdcw dependence to the feature, but the zdcw dependence of the west-side reference makes it possible that the dependence is NOT due to ion backflow increases with luminosity.  Another way to consider this data is to subtract off the west side from the east side, which looks like this:



There is no zdcw dependence of the feature apparent above the visible fluctuations. So this may not be an ion backflow artifact.

-Gene