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 AcdTkr1VetoSigmaHit and VetoHit1

AcdTkr1VetoSigmaHit is the probability an estimator of the "best" hit vetoing the track, expressed in "sigma".     This is the sum in quadrature of contributions from the miss distance between the track and the tile or ribbon and the signal size being less that 1 MIP.  For MIP-like signals along the track projection both terms are 0, so this variable is 0.  If the track missed the tile  this variable is postive, likewise if the signal is less than 1 MIP.

 
Same plot, but with the X-axis remapped with log10( AcdTkr1VetoSigmaHit + 0.01)

Note the small peak in the signal distribution around 7-9 is from low energy deposits in the tile pointed to by the track.
For completeness here is a plot of  AcdTkr1VetoSigmaHit v. AcdTkr1ActiveDist

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Same plot, but the X-axis remapped with log10(VetoHit1+0.01)

 
Note Again, note the small peak in the signal distribution around 7-9 is from low energy deposits in the tile pointed to by the track. 
 

  AcdTkr1VetoSigmaGap and VetoGap1

 AcdTkr1VetoSigmaGap is the probability that an estimator of the track went in passing through a gap in the ACD, expressed in "sigma".     This is the sum in quadrature of two contributions geometrical  : the geometrical       calculations of the track going in 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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We can also select the best association by looping over all tracks instead of just using the best track.
Here is a plot of that variable, AcdTkrVetoSigmaHit: 

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With these variables (and some familiar ones) we can make a Classification Tree.

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).  Image Added

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AcdEng15Sc

 
Just to give some numbers, here are numbers for 3 different values of cutting on the CT output. 

 Basically, these numbers correspond to either 15% reduction in residual background at given signal efficiency or 3-4% increase in signal efficiency at given background.