[d2n-analysis-talk] Positron dilution

[posik at jlab.org]

Hi all,

   So I spent a fair amount of today trying to get a positron dilution
factor. The first thing that I did was to use a run set (runs
2199-2212) where the BigBite magnet was in positive polarity mode,
meaning we are detecting positrons. I applied our good electron cuts to
this positive polarity set and the cuts look fine. I then went ahead
and computed a dilution factor.

  D = 1 - (N+/N-)*(Q-/Q+),
 where N+ are the total events counted with BB set to positive polarity
       Q+ is the total charge for the run set with BB set to positive
polarity
       N- are the total events counted with BB set to negative polarity
       Q- is the total charge for the run set with BB set to negative
polarity

 This dilution factor can be seen  as a function of x below
(pos_outrun_dilution.eps)

 After talking to Zein-Eddine, because we only have one positive polarity
setting at 4.7GeV, and I could not find any at 5-pass, he suggested
looking at the positrons and electrons with in a common run set. For this
I used the BB negative polarity setting corresponding to runs 2024-2195.
I chose electrons using the BB.optics.charge[]==-1 cut and positrons
using BB.optics.charge[]==1 cut. This time for the Dilution factor, since
the total charge for all these runs is the same for positrons and
electrons, the dilution factor is:

  D' = 1-N+/N-
A plot of D' can be see below (pos_inrun_dilution.eps)

 The two dilution factors differ quite a bit at low x and agree at higher x.
 I suspect that the shape change at lower x is due to the BB field bending
positrons away from the detectors when it is in negative polarity mode.
 I am uncertain what dilution factor should be included in the
asymmetries? Or if there is another way to go about getting the positron
contamination?

Thanks
Matt
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