Hypothesis: SpaceCharge could affect drift velocity by applying a force on the drifting electrons in the z direction (while such a force would be z-dependeny, SpaceCharge builds up more near the central membrane than endcaps, so the effect is not balance when averaged over all z).

N.B. Importantly, this has been postulated not to be an issue because the TPC is operated at the peak of the drift velocity vs. drift electric field curve, so modifications to the drift field (z components of E fields) should have minimal impact.

In the plots below: all DVs in [cm/us]; all scalers in [Hz]; all times in [sec] (unix time).

Run 9 pp500:

DV vs. zdcx

Red is binned average of black points.

Some correlation might seem to be there, but it isn't certain. Luminosity increased towards the end of pp500 running as the collider operations improved, but environmental conditions also changed and may explain the late drop in DV. Here are DV and zdcx vs. time:

Run 10 AuAu200:

Blue is east <DV>, red is west <DV>, black is un-binned (east + west)/2.

DV vs. zdcx:

DV vs. bbcx:

And for completeness, west DV vs. t:

Conclusion: not enough evidence to conclude that there's a dependence.

-Gene