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["Van Buren, Gene" <gene AT bnl.gov>]

Hi, Babu and TPC-ers

A quick re-cap on what we did for the Run 21 dAu200 TPC SpaceCharge calibration:

Very large and asymmetric (yellow vs. blue) beam backgrounds were observed for the first 3 days of data taking (days 180-182, worst on days 181-182) when the collider operators tried to work with beam crossing angles at STAR. This improved during days 183-188.

Coincidentally, Babu's processing of those data also showed an abnormally _symmetric_ east vs. west SpaceCharge distortion for days 180-182 (it _should_ be asymmetric in dAu). So the assumption was that the backgrounds made for more symmetric charge distributions. To handle this properly, one needs to look at how the charge is radially distributed differently during days 180-182, but this was more than Babu had time to do, so we simply approximated by allowing for an east-west symmetry of SpaceCharge under the usual radial distribution and pressed forward.

A good check of how this is performing comes from looking at charge-splitting of the (anti-)proton mass^2 in the BTOF. And Chenliang just showed me his findings today. Please see the attached plot made by him:

1) For days 183-188, everything looks okay (perhaps a little low, but that isn't due to SpaceCharge which causes splitting)

2) For days 180-182, there is notable mass^2 splitting (worst on days 181-182)

This splitting was not seen by Chenliang in the AuAu17.3 calibration data.

This implies that our SpaceCharge approximation for days 180-182 of dAu200 is poor. We can press forward with producing this data, but I will caution the analyzers that data from 180-182 is not great and may be worth skipping. We should investigate the radial distribution of charge in that data and try to improve the SpaceCharge (notably more work) for a subsequent re-production, which we will probably want to do eventually anyhow when we have an updated TPC alignment in place.

Thanks,

-Gene