[G8b_run] Tagger Sag and Proton Momentum Corrections

[Charles Hanretty]

Dear colleagues,

We apologize for the delay with regard to the requests from the last g8b 
meeting. I've invested more efforts to estimate the required photon 
energy correction due to the tagger sag using the kinematic fitter. We have
also double-checked again our pull distributions for the double-pion reaction 
gp->ppi+pi- (all particles detected) including error adjustments, and 
(hopefully) finalized the proton momentum corrections.

The first page of the pdf file in the attachment shows again the tagger sag; 
this time, the photon energy is plotted versus the (fitted) mean of the 
photon energy pull. The typical structure that we have been discussing for weeks
is clearly visible. The lower left plot shows the photon pull integrated over
all energies (essentially the projection of the upper plot onto the y-axis).
Ignoring a possible energy dependence of the tagger sag, we tried to center the
fairly symmetric pull distribution in order to get a feeling for the required
energy correction. The original (black) distribution (no tagger photon correction) 
requires approximately a 0.2% energy correction to become the (much better 
centered) blue distribution. The mean and sigma are given on the slide 
following the same color convention.

Now we went back to the proton momentum corrections that were previously 
determined without a photon energy correction. This is shown on the second page.
The mean of the (global) proton momentum pull has become worse and shifted from
almost zero to about 0.18. To correct for this shift, an additional 0.4% proton 
momentum correction would be required. This is much less than the original 
proton momentum correction, which is of the order of a few percent.

So far, we have not applied any pion momentum correction. The status of the 
pion pulls is shown on the third page. While the error adjustments (pull 
widths) are all very acceptable (the comparison with Mike Williams values for 
the g11 data is shown on the fourth page), the pion means are somewhat off 
center. The displacement is of the same order as for the proton and only a very 
small correction would be required to center them. Unfortunately, this is a 
very tricky business. Further proton corrections shift the pions farther away 
from zero; we observe the same effect with the proton when we try to correct 
the pion momentum.

For this reason, we think that the current status of all the corrections
seems to indicate a very good compromise: a very small 0.2% photon energy 
correction (a few MeV without any energy dependence) and proton momentum 
corrections as determined before. Given the small energy correction, we feel 
that ignoring the energy dependence is justified for energies below 2.2 GeV, 
which covers the important photon polarization of the g8b data.

Thoughts?

-Chuck and Volker
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