The FTPC's are not mounted exactly at x=0,y=0. There is a slight rotation about the mounting point.The long lever arm of the FTPC tracks to the primary vertex increases the effect of this rotation which results in an offset of several millimeters in the (x,y) plane when the FTPC tracks are projected to the primary vertex.
We determine the rotation calibration by fitting a Gaussian to the Ftpc vs. Tpc vertex QA histograms projections. This procedure is described in STAR FTPC Software and Calibration Overview (pages 21-23).
In the 2006 run, pileup made it differcult to apply the Gaussian fit to these histogram projections. The pileup effect is visible in the QA impact parameter histograms:
|FTPC East||FTPC West|
After track reconstruction, StFtpcTrackMaker calculates the primary vertex position using the FTPC fitted tracks which pass the dca cut defined by the FTPC tracking parameter maxVertexDca. Historically maxVertexDca = 100. There was always one peak in the FTPC vs. TPC vertex histograms. Then, in the 2006 pp run, many peaks appear due to pile-up.
Looking at the FTPC impact parameter histograms above for all run years, it looks like setting maxVertexDca = 2 will eliminate most of the pile-up in the 2006 pp run and will have minimal effect on previous runs.
Note: The primary vertex position is calculated separately for FTPC East and FTPC West. These two vertices are stored in StCalibrationVertex with vertex type = kFtpcEastCalVtxId and kFtpcWestCalVtxId respectively. In St_QA_Maker the differences between the TPC determined primary vertex and the FTPC East/West determined primary vertex are histogrammed for each event. These histograms (StEQaVtxFtpcETpcXY and StEQaVtxFtpcWTpcXY) are used to calculate the slight physical rotation around the FTPC mounting points.
I reconstructed the 2006 pp run file st_physics_7098053_raw_1030014.daq for maxDcaVertex (in StarDb/ftpc/ftpcTrackingPars.C) = 100, 10, 5 and 2. Here are the resulting Ftpc vs. Tpc x,y projections for Ftpc East and Ftpc West with a Gaussian fit for these four settings of maxDcaVertex:
set maxDcaVertex = 2 in StarDb/ftpc/ftpcTrackingPars.C