TPC Anode voltage effect on W reco efficiency

 

Please disregard this post, the important quantity is the Luminosity dependence which is studied here.

 

TPC Anode voltage effect on W reco efficiency

During the pp500 portion of Run 9 the Inner TPC Anode high voltages were lowered from 1170V -> 1135V.  This change occured before run R10094003 (noted here).  The embedding sample should account for this change in HV, and this change in efficiency between these 2 time periods is what is studied below.  Only W+ values are shown below because the statistics are larger. 

Embedding efficiency: (sample split into to sets one before and one after the HV change)

Inner Anode HV Run range W+ track effic W+ total effic
1170 10085131-10088070 0.81 +/- 0.01 0.57 +/- 0.02
  10088071-10093084 0.80 +/- 0.01 0.57 +/- 0.02
1135 10094007-10099057 0.78 +/- 0.01 0.55 +/- 0.01
  10099066-10100179 0.75 +/- 0.02 0.52 +/- 0.02
  10100180-10103042 0.75 +/- 0.01 0.52 +/- 0.02
Ratio (1170/1135) Average   1.08 +/-  0.03

Note: the difference in the total efficiency is almost completely due to a drop in the tracking efficiency (as expected).

 

Data:

To estimate the difference in efficiency before and after the HV change calculate the x-section multiplied by the efficiency as

formula

Inner Anode HV N_raw N_bkgd L sigma*epsilon
1170 185 15 4.2 40.5 +/- 3.2
1135 321 29 9.0 32.4 +/- 2.0
Ratio (1170/1135) 0.58 0.52 0.47 1.25 +/- 0.12

 

Attached below are some plots of the track QA variables which are cut on in the analysis like nFit points, nFit/nPoss, first hit R, last hit R.  The distribuations are of events that pass all the W cuts and have a BTOW 2x2 cluster ET > 28 GeV.  Each page contains 4 plots.  The upper row are the data and the lower row are the embedding; the left column is inner anode HV = 1170 and the right column is inner anode HV = 1135. 

The data and embedding distributions match fairly well for the track QA observables.  Also the distributions don't change drastically between the two HV settings, so the question becomes what causes this drop in tracking efficiency seen in both the embedding and data?

Two other interesting distributions included there are the track PT and track chi^2 plots that fairly different in the embedding and data.  The Pt distribution is much wider in the data than in the embedding, and the chi^2 is larger for the embedding than the data.  The wider Pt distribution could have an effect on the tracking efficiency due to the hard cut on Pt > 10 GeV we impose in the analysis. 

 

Other thoughts:

Since sigma is the same for both HV values the ratio of sigma*epsilon is the same as the ratio of the efficiencies (Note: statistical uncertainties shown).  So for the embedding the efficiency is ~8% higher when using the higher HV, and in the data the efficiency is ~25% higher when using the higher HV.  The statistical precision of the data is not so great, but maybe there are some conclusions to draw from this?  A couple options if you take this difference seriously:

1) If the embedding efficiency is correct for anode HV = 1170, then we should use a smaller efficiency for the lower HV portion of the run and our measured x-section will go up.

2) If the embedding efficiency is correct for anode HV = 1135, then we should use a larger efficiency for the higher HV portion of the run and our measured x-section will go down.

3) Or it is somewhere in the middle, and its not clear what to do.