AuAu data was taken at 200 GeV during 2007 (Run VII). We have Full Field (FF) and Reversed Full Field (RFF) data. During data processing, we will again use the event-by-event SpaceCharge + GridLeak correction method. As a reminder, this means:

1. A scaler-based correction is used as an initial guess for a prepass to determine more accurately the distortion correction needed for a particular file.
2. The prepass value is used to start the production pass.
3. Once enough statistics are built up, the event-by-event method kicks in.
4. If there is a long gap in time, or a long series of low statistics events, we revert to the prepass value.

In this scenario, it is not imperative that the scaler-based calibration of the SpaceCharge-to-scaler (we call it SC herein) ratio is perfect. However, it is imperative to get the value correlation the GridLeak to SpaceCharge (we call it GL) correct. It is also beneficial to the event-by-event method to have SC as close to correct as possible as it helps expedite convergence on the best solution.

It appears that the SpaceCharge & GridLeak are larger than they have ever been before. At this point (and we have seen this in the past with the highest luminosities in CuCu), we begin to have track splitting due to the GridLeak distortion. Therefore, the calibration cannot begin with zero corrections. So I took a stab at a few guesses based on old AuAu data for my initial starting points. It also became clear that the ZDC coincidence rate (ZDCx) appears to have the tightest correlation with the distortions.

In the end, the calibration converged quite well for both FF and RFF data. Of note, the values of SC (corrected for the use of ZDCx instead of the ZDC east+west sum we used in 2004) and GL stayed near what has been measured in the past, which is good confirmation of our understanding of these effects. Additionally, the FF and RFF values match quite well with each other.

The low luminosity data turned out to be important in nailing down the behavior of the ionization distortions. It turned out there was a small systematic problem in the Calib_SC_GL.C macro which became noticeable because of this low luminosity data (due to using histogram bins and losing some information) which was introducing some of the offset at zero luminosity, and it is now fixed. One conclusion from the good match between the FF and RFF offset is that there does appear to be a real offset at low luminosity, and that offset is such that there is no distortion below a certain luminosity (ZDCx of approximately 500 Hz in AuAu).

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Full Field

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Reversed Full Field

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Gene Van Buren