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AcdTkr1VetoSigmaGap is the probability that the track went in a gap in the ACD, expressed in "sigma". This is the sum in quadrature of two contributions geometrical : the geometrical
calculations of the gap and the chance of seeing no signal from the gap. For high energy tracks that pass into the corner gaps of the ACD it is 0. For tracks going into ribbons it is 1.5 (which represents the fact that we do expect some signal from the ribbons, albiet a small one).
Here is a plot of the low end of AcdTkr1VetoSigmaGap for all events:
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This plot show the performance of the CT (in red) relative to the pass 6 cuts (3 black circles). The prefilter performances are also shown.
For comparison the CT performance without any prefilters is also show (in blue). 
Just to give some numbers, here are numbers for 3 difference different values of cutting on the CT output.
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Basically, these numbers correspond to either 15% reduction in residual background at given signal efficiency or 3-4% increse increase in signal efficiency at give given background.