Some findings from Ben Kimelman's studies on correction for distortions due to SpaceCharge fluctuations in AuAu27...

Red: uncorrected <signed DCA> [cm] vs. time [sec] for ~20k events from run 19132074
Blue: corrected <signed DCA> [cm] vs. time [sec] for the same data using an ion drift velocity of 279 cm/sec

(note: red is plotted on top of blue, so where blue isn't visible it's data are identical to red, i.e. the correction there is zero)



Same data overlaid modulo 2.0 seconds (to increase statistics per unit time):



Conclusion: 279 cm/sec is clearly too fast for the ion drift velocity as the distortion does not plateau and clearly needs corrected for a longer period (closer to ~1.0 seconds, implying an ion drift velocity closer to ~210 cm/sec) after the deposition of charge in the TPC.

Jim Thomas comment on these:
"Your data would suggest that Ar⁺ is the ion that is carrying the charge over long distance. (A different conclusion than over short distances.) If you believe the pre-turn-of-the-century literature, then Ar⁺ has a mobility of 1.54 cm²/(volt*sec) which is pretty close to your estimate of 210 cm/sec velocity. So on the short scale, the charge may be carried by Methane and Ar⁺⁺, but on the long distance its Ar⁺."

Additional note: because DAQ data is used to determine when and how much charge is deposited into the TPC (see this documentation), this currently has not means of determining charge that came before the first event of a run. However, up to ~1 second of data may be affected by the charge that was deposited immediately before the run started, but remains uncorrected. This is visible in the first plot above where the first ~0.7 seconds shows a distortion with no correction.

-Gene