> > Steve, can we be absolutely sure that the BB vertex is "off" by 2 cm.? > Or the ZDC vertex for that matter? Couldn't it be that the TPM1 vertex > is off by 2 cm. (or 4 or 6 or ...)? In short, what is it that makes > everyone so sure that the TPM1 vertex is the "best" and should always > be right? I've previously suggested trying to extract a primary > vertex position from all avaliable primary vertex meassuearments in a > given event. How say you (all) on that? The only detector that can possible give an absolute vertex is the TPM1 - all others are subject to unknown calibrations offsets (2 cm is only 60 psec) e.g temp change etc can easily do that. The BB, ZDC do not have a commen external signal (e.g. laser with a common source and calibrated feed-lines to them. It is of course possible to get the wrong vertex from TPM1 - the recent discovery of the wrong pitch .39 instead of .40 shows this - on the other hand you can estimate from the survey and its accuracy how far off you can be - and it is probably < 1 cm >
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