Impact Parameter Distributions at Primary Vertex

A. C. Saulys

BNL

Using the DEV version of library

• Impact parameter distribution using globtrk.impact for negative tracks
• Impact parameter distribution using globtrk.impact for negative tracks with a pt cut gt 500 MeV/c
• Impact parameter distribution using globtrk.impact for negative tracks with a pt cut lt 500 Mev/c

Using my version of library and the Kalman filter for globtrk

• Impact parameter distribution using globtrk.impact for negative tracks
• Impact parameter distribution using globtrk.impact for negative tracks with a pt cut gt 500 MeV/c
• Impact parameter distribution using globtrk.impact for negative tracks with a pt cut lt 500 Mev/c

Using my version of library and a constrained fit at primary vertex

• Chisquare distribution of impact parameter at primary vertex
  Here is a plot of the chisquare distribution of globtrk using the covariance matrix from the Kalman filter and using Victor's constrained fit code to require the track to go through the primary vertex.
• Impact parameter distribution at the primary vertex with chisq.lt.3.0
  After the cut there are no tracks beyond impact parameter of 4.0. This is to be compared with the next plot.
• Impact parameter distribution for all negative globtrk.
  Here there is a long tail past 4.0 corresponding to the secondaries.
  

From this VERY PRELIMINARY study, I conclude that cutting on the chisquare is a very good selection criterion for primary tracks. The advantage in using this type of cut is that it is momentum independent, where the impact distribution itself is not as shown in the previous plots above.