OFC West possible distortion

There remains a possible distortion due to a potential short in the OFC west as well. We see a bimodal pattern of 0 or 80 excess nanoAmps coming out of the OFC West field cage resistor chain (it has been there since the start of the 2005 run). That corresponds to a 0.38 MOhm short (420nA = 2 MOhms). The corresponding distortion depends on the location of the electrical short. The plot shown here is the distortion in azimuth (or rPhi) at the outermost TPC padrow near the sector boundaries (r=195 cm, the pads are closer to the OFC near the sector boundaries) due to such a short between different possible field cage rings:

In terms of momentum distortion, a 1mm distortion at the outermost padrows would cause a sagitta bias of perhaps about 0.5mm for global tracks (and even less for primary tracks), corresponding to an error in pt in full field data of approximately 0.006 * pt [GeV/c] (or 0.6% per GeV/c of pt). This is certainly at the level where it is worthwhile to try to fix the distortion if we can figure out where the short is. It is also at the level where we should be able to see with the lasers perhaps to within 50cm where the short is.

Just as a further point of reference, the plot for radius = 189cm, corresponding to the radius of the outermost padrow in the middle of a sector (its furthest point from the OFC) can be found here.

This possible distortion remains uninvestigated at this time.

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Gene Van Buren
gene@bnl.gov