[Clascomment] OPT-IN:Differential Photoproduction Cross Sections for the Sigma0(1385), Lambda(1405), and Lambda(1520)

[Reinhard Schumacher]

Hello Volker,

	You did indeed spot an overlooked problem in the paper with
the lowest W-bin result for the L(1520) state.  We have fixed it in
the revised draft. Thank you for looking at the results with "fresh
eyes" and finding this. See below.


On 05/15/2013 09:54 PM, Volker Burkert wrote:
> I am sending this again as my first attempt at sending it to the clascomment repository apparently
> had failed. There is nothing new in here that you have not seen before, I am sending it just for the record.
>
> Dear Kei and Reinhard,
> This is a very interesting paper and it is good to see how much information can be extracted
> from the high statistics of the G11 run. I don’t want to comment on the editorial aspects here
> but I came across one potential flaw in one of the results. As is commented in the paper the
> cross section for the K-Λ(1520) shows an unexpectedly sharp drop for the first bin in W
> compared to the next higher W bin. It also undershoots by a factor 2 the theory curve as
> well as the LEPS data, both of which are in good agreement with our data for the next higher
> bins in W. I have the suspicion that acceptance may not have been correctly taken into account
> for this first W bin. The bin width is 100MeV and extends from W=1950 to 2050 MeV.
>
> However, the nominal threshold for the process γ + p → K+ + Λ(1520) is at
> Wthr = 2013.2 MeV [= 493.7 + 1519.5  (peak)], or if I subtract  Γ/2 = 7.8 MeV [half of full width of Λ(1520)],
> it is still Wthr=2005.4 MeV.  This means that the production threshold is about 55MeV above
> the lower edge of the lowest W bin, and the bin is thus only about 45% filled
> (neglecting resolution effects).  If the acceptance for this bin was calculated the same way as
> for the other bins, i.e. acceptance = Monte Carlo events accumulated in bin/ nominal bin width,
> the cross section will be about 50% low.
>
> I also noted that this lowest W bin is not included in the total cross section graph in Fig. 11,
> where the bin center would have ended up just below threshold.This is, of course, a well-known
> problem with edge bins that go across the production threshold, and our very first paper
> (phi photoproduction) showed the very same problem for the last bin in t. Fortunately,
> it was detected and fixed before publication.
>
> These kind of problems should have been caught
> in the PWG review, and the fact that the cross section is 50% lower than the LEPS data show
> may have caught the attention of the ad hoc committee as well, and perhaps all this was
> scrutinized and discussed already, in which case I apologize for bringing it up again.
> However, the effect is so striking and seems highly unphysical, so I felt necessary to bring
> it up as a possible issue.
>
The fault for not spotting this sooner is ours alone, I think, and we 
thank you for being suspicious about it.  Not only was that W bin not 
fully populated with data because it spans the L(1520) threshold, but 
our acceptance just above the threshold is also very small.  This was 
very easy to remedy, however, since we are able to select a restricted 
range of W where we have both data and acceptance.  The renormalization 
factor was about 2.6 when we trim the bin to be 2.02 to 2.05 GeV, or 30 
MeV wide instead of 100 MeV wide.  By varying this selection somewhat to 
test its effect on the result, we estimate that this bin has an extra 
overall normalization uncertainty of 36%.  We give this uncertainty in 
the text.  As you can see in Figure 10, the agreement between CLAS and 
LEPS is now quite good.  The text has been modified to reflect this. 
Also, the numerical results presented in Table III
have been revised to show the special treatment of the first L(1520)
bin.

> There is another thing that look suspicious to me, the drop by a
> factor 4 of the K-Λ(1405) cross section for the lowest W bin in the Σ0π0 channel around 90deg.
> This drop occurs from one angle bin to the next. However, I don’t suggest an explanation for
> such a behavior, but it would be good to make a comment in the paper that this was scrutinized
> and found to be correct as far as the analysis is concerned.
>
> Volker
>
>
We did take another look at all the fits that went into this section of 
results.  Individually they all look OK;  there was no mistake here.  We 
mention this check in the text, but don't try to explain why the cross 
section might do this.

Reinhard and Kei


___________________________________________________________________
Reinhard Schumacher         Department of Physics, 5000 Forbes Ave.
Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A.
phone: 412-268-5177         web: www-meg.phys.cmu.edu/~schumach
___________________________________________________________________