[G8b_run] Tagger Sag and Proton Momentum Corrections

[Ken Livingston]

Hi Chuck, Volker, Mike,

Thanks again for all the hard work on this. Lets have a conference call 
on thursday as arranged to discuss  where we are now.

A couple of questions still come to mind:

1. The pulls are in general all quite a bit higher than those from Mike 
Williams thesis (apart from the photon energy - presumably since the g10 
correction is already applied in the cooking now).  Why is that? 

2. Why does correcting the pion make things worse, and can we justify 
ignoring this.

3. What are we going to do about Kaons ?

Hopefully we can agree to get this finalised. We can move on to redoing 
the polarization tables and working up the energy scale with the 
polarization systematics.

Cheers,
Ken



Charles Hanretty wrote:
> 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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-- 
=======================================================
Ken Livingston          

Dept. of Physics & Astronomy,        Tel: +44 141 330 6428
University of Glasgow,               Fax: +44 141 330 5889
Glasgow G12 8QQ.
Scotland. UK.
=======================================================