[d2n-analysis-talk] Pion Rejection Factors in the LHRS Pion Rejector

[David Flay]

Hi all,

I wanted to provide you with an update on the pion rejection analysis.

For the study in the Pion Rejector, the procedure is as follows:

using the same standard cuts I outlined in the GC pion rejection study, we
choose a cut in the GC and see how many events pass that cut as seen in
the E/p distribution.  Then, we compare that to the E/p distribution
without the GC cut.  Then, we integrate over both distributions from E/p =
0 to 0.5.  We then consider the ratio r of the number of events (Ni) of
the distribution without the GC cut to the number of events (Nf) of the
distribution with the GC cut.  This tells us the pion rejection factor for
this particular E/p cut (the events excluded by a cut on E/p = 0.5).  So,
we have:

r = Ni/Nf = pion rejection factor (indicates the amount of pions removed
by a cut on E/p)

=> this is why we integrate over the region E/p = (0,0.5).

I have carried this procedure out for 3 kinematics: p = 0.6, 1.20, 1.70,
at 7 different E/p cut positions, as outlined in the first three plots
attached.  Also attached are plots of E/p for each kinematic, displaying
the E/p distribution before and after a GC cut (GC cut distribution shown
in red).  I will provide a more concise and detailed analysis at our next
analysis meeting.

I believe that the trend seen in plot 1 is due to the fact that the main
peak of E/p (for electrons, shown in red) is lower than the higher
kinematics, so the pion rejection factor tends to decrease faster than
seen at higher kinematics for a given cut on E/p.  Subsequently, I believe
this is tied to the E/p drift effect ( the main peak position seems to
have a slight momentum dependence) I've been seeing across the whole
kinematic range.  I will have to adjust for this momentum dependence in
some way.

The reduced pion rejection factor at higher kinematics is reasonable as
there are less pions at higher p.

Overall, I believe that these numbers seem reasonable so far, while the
sharp drop in pion rejection factor as a function of E/p cut for p = 0.6
merely reflects not only how bad the pion contamination really is, but
also illuminates the effects of the momentum dependence of the E/p peak
position.  Subsequently, I also believe this is why the cut pion rejection
factor doesn't change as much as a function of E/p cut position at higher
p, since the electron peak has much better separation from the pion peak.

Dave


-------------------------------------------------
David Flay
Physics Department
Temple University
Philadelphia, PA 19122

office: Barton Hall, BA319
phone: (215) 204-1331

e-mail: flay at jlab.org
              david.flay at temple.edu

website: http://www.jlab.org/~flay
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