[Clas_hadron] DNP presentation for Mini-Symposium

Fri Oct 18 19:31:13 EDT 2013

Andy, Natalie, et. al.,

Given that the comments relating to my analysis (slides 12 and 13) take 
up a significant portion of this last message, I suppose I better step 
in and defend them.  Although I broadly agree with the arguments 
regarding the Lambda recoil polarisation, there are a few things I 
should point out regarding my FROST results.

1) The low energy bins of the 'S' measurement show very good agreement 
with the g8b data shown, in my own humble opinion. However, these plots 
seem to be hiding under the higher energy bins on slide 12 of Natalie's 
pdf, and I can't get at them in my non-proprietary pdf viewer.  They're 
from my Baryons2013 slides, and these are available here (slides 19 [low 
energy] and 20 [high energy]); 
http://nuclear.gla.ac.uk/~stuartf/talks/SFegan_Baryons2013.pdf. The main 
problem with the higher energy bins relates to the methods used to 
account for the nature of the FROST target, which is pretty far removed 
from the ideal free proton target, and I'll come to that in a moment.

2) The g8b results shown for comparison with my 'S' measurements are 
themselves preliminary, and pre-date the g8b analysis currently under 
review.  The old g8b results were all I had available when I produced 
these plots for Baryons, but I should have this rectified by the next 
collaboration meeting, where I intend to present a further update to my 
FROST work.

3) These results were the first major update to the analysis since my 
thesis, coming after the availability of detailed photon polarisation 
tables, and a bit of work harmonising the particle and channel 
identification with the aforementioned g8b review. With very little new 
work done on the observable extraction side of things, these interim 
results demonstrate a need to revise the rest of the analysis, whose 
issues were obscured until this recent availability of a much cleaner 
hyperon event sample, and smaller systematic errors from photon beam 
polarisation.

4) The nature of the FROST target material means a significant quantity 
of bound nucleon events contribute to the value of "P_gamma S" extracted 
via the binned asymmetry method (where P_gamma is, of course, photon 
polarisation).  Accounting for this contribution has been one of the key 
difficulties in analysing data from this target, and (see point 3 above) 
the methods of accounting for this, were yet to be re-examined in any 
detail when those plots were produced (although the values relating to 
this effect were recalculated using the new event samples as described 
in chapter 6.3 of my thesis).

5) I think you can guess by now that the source of the "new analysis" 
was me.  The results differ, but only because of everything I have 
stated above.  The plots were produced by running the new hyperon event 
samples through the same ROOT macro I used for my thesis, with the new 
photon polarisation tables and recalculated estimates of the dilution 
effect due to bound nucleon events.

So that's about the status of my side of things, hope that's provided 
some useful follow-up information should any questions come up about 
this during the talk.  As I say, I'll be presenting a further update to 
this work in November at the next Hadron Spectroscopy meeting.

Cheers,
Stuart

On 18/10/2013 18:06, A.M. Sandorfi wrote:
> 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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