Star-tpc-l mailing list

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

Writing down my thoughts....

Chenliang doesn't see a large ZDC rate dependence in mass2 splitting even without SpaceCharge corrections, but the dependence is there at a small level (Chenliang sent me a plot showing that the proton splitting goes from about 0.012 at low luminosity to about 0.006 at high luminosity in the SpaceCharge-uncorrected sample).

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Additional observation:

I compared momenta directly between the two recent 19.6 GeV calibration samples using h-/h+, dividing one sector's h-/h+ in the newer sample (w/o SpaceCharge correction) with the h-/h+ in the older (with SpaceCharge correction). I found that the momentum changes were essentially constant from one sector to another (no surprise as the SpaceCharge correction used is symmetric in eta and phi), and that the scale of the change to primary track momenta due solely to the SpaceCharge correction... (blue dots here)....

    https://drupal.star.bnl.gov/STAR/system/files/userfiles/17/image/Calibrations/TPC_Alignment/hmhp/Run19/hminus_hplus_exponentParameters_19p6_nosc_over_sc.png

...is an order of magnitude smaller than the scale of sector-to-sector variations I see within any of the datasets, e.g.

    https://drupal.star.bnl.gov/STAR/system/files/userfiles/17/image/Calibrations/TPC_Alignment/hmhp/Run19/hminus_hplus_exponentParameters_19p6_nosc.png

Taken together with Chenliang's findings for the 19.6 GeV with and without SpaceCharge, I think it is fair to conclude that the actual SpaceCharge distortion itself pushes the antiproton mass down a little and the proton mass up a little, such that the difference of antiproton mass2 minus proton mass2 when integrated over the whole detector gets a little smaller if we _don't_ correct for it. Correcting for it pushes them back apart in this dataset.

The issue remains that we have a notable offset in mass2 splitting at the lowest luminosities, in SpaceCharge-corrected higher luminosities, and there is also significant variation sector-to-sector in all datasets.

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An encompassing topic has to be whether this amount of mass2 splitting is acceptable for analyses. To give context, Emmy Duckworth has been discussing mass2 splitting in AuAu27 and isobar data from Run 18, which had low luminosity, and which is likely also linked to remaining TPC mis-alignments from the pre-iTPC era. The scale of Emmy's measurements are [-0.010 - +0.001] GeV/c^2, depending on where in the TPC one looks - see the attached plot (I think Emmy does protons minus antiprotons). Chenliang is showing results at the 0.006 GeV/c^2 level. So these two different years' issues are of comparable scale.

In brief: if we don't resolve the mass2 splitting, we're at a similar level to past performance, and that implies that a goal of the iTPC effort, namely to have improved momentum resolution and systematics, will not be achieved. If we are considering accepting it, we should try to understand at what level important analyses are impacted.

Different conclusions may be drawn from the issues we're having with track DCAs in the FXT data.

Thanks,

-Gene