dAu east-west DCA asymmetry

Looking at the signed DCA of tracks on the east half of the TPC minus the west half of the TPC for Run 8 dAu data gives the following results using the calibrations for SpaceCharge and GridLeak inserted into the DB on 2009-05-27 (see here). This is shown as a function of luminosity as measured by the amount of space charge. Black points are the difference using several hundred events to get the values of dca east and dca west separately, and the red points are from a profile histogram of the black points:


The above plot use an ewratio of 1.202 for the ratio of charge in the east TPC versus the west TPC caused by the asymmetry of the dAu collisions. The result shows a luminosity dependence in this difference consistent with requiring a larger value for this ewratio. My estimate from the data is that the increase needs to be a mere 1.2% increase on the east and 1.2% decrease on the west, or ewratio=1.231. I tried re-running the calibrations with that ratio and found an improved result:

Before finalizing this re-calibration, I will wait until we better understand any east-west asymmetries seen in the Run 8 pp data.