[Clas_hadron] DNP presentation for Mini-Symposium

[A.M. Sandorfi]

Hi Natalie,

You have an interesting talk. I have a few comments beyond Volker's:

Slide 4: I see you still love the historical table, even though it's
incomplete.

Slide 5: The wording of your footnote "^" is not correct. You can point out
that the measurements denoted with a "^" were obtained in
double-polarization experiments, which can be more prone to systematics than
the other way of measuring the same matrix element, with a
single-polarization experiment. But they are both measurements of the square
of a reduced matrix element. The designation of "direct" has no meaning.

Slides 9 through 14: when you compare data from multiple sources, plotted
with different symbols, you need to include a legend on each slide so the
audience can quickly understand what you are comparing. Some times you have
this in words somewhere on the slide, which is not as clear, but often there
is no hint at all.

Slide 12: Although the errors are larger, the red g9a points are generally
in poor agreement with g8. There is a likely reason for this. Your first
bullet states that you have averaged +z and -z target polarizations. An
experimental asymmetry measurement constructed as (par-perp)/(par+perp)
contains 11 different observables, one of which is "S", the beam asymmetry
you are after. All of the others are multiplied by a component of the Lambda
recoil polarization, for example Pt(z)*Pr(z')*Lz'. For either parallel or
perp photon polarizations, when you flip the sign of the target polarization
Pt(z), the signs of the two recoil components Pr(x') and Pr(z') also flip.
So when you average the yields from +z and -z target polarizations, terms
like the above one in Lz' add; they DO NOT CANCEL OUT. The
(par-perp)/(par+perp) ratio will reduce to "S" only if the Lambda => pi-p
decays are completely integrated so that the recoil information,
Pr(x',y',z'), is thrown away. That depends on modeling detector acceptances,
holes in CLAS, etc. and certainly has significant systematic issues. If you
want to show slide 12, you must qualify the first bullet with a statement
about the additional systematics involved in averaging over the recoil.
(If you extracted "S" by fitting cos(2phi) moments, then it's a little more
complicated. The unwanted terms can still drop out, but only if the
averaging over the Lambda decays is done just right.)

Slide 13: this is presumably a new analysis, since the results differ from
figure 7.8 of Fegan's thesis. The source should be quoted or referenced
somehow. Depending upon how this was done, it is susceptible to the same
problems as I mentioned above. Changing the sign of the target polarization
changes the recoil polarization components, so that the results for G depend
critically on a careful average over the pi-p decays so as to intentionally
loose the recoil information.

Slide 14: this is missing the g8 results for T. These should also be plotted
here. According to slide 2, you are summarizing both. I realize that they
agree with GRAAL and disagree with your g9a analyses for many energy bins.
That I suspect has to do with the above recoil issue. The Lambda decay has
to be detected to pull the channel out of the background. But once detected,
the recoil information is in the data stream, whether or not one chooses to
look. Once in hand, throwing it away becomes tricky. (Ironically, there may
be less uncertainty in analyzing the recoil and using it, than there is in
trying to throw it ALL away.) In any case, these issues should be aired.

Andy



On 10/17/13 7:19 PM, "natalie at jlab.org" <natalie at jlab.org> wrote:

> Hi,
> I have linked my presentation for next week's mini-symposium at the DNP
> meeting on Thursday. I was asked to give an overview talk of kaon
> photoproduction at CLAS. Please let me know if you have any comments or
> suggestions.
> 
> http://www.jlab.org/Hall-B/secure/g9/natalie/dnp_minisym_kaonphoto_walford.pdf
> 
> Thanks,
> Natalie
> 
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