[Clascomment] OPT-IN: Differential cross section of gamma n --> K+ Sigma- on bound neutron with incident photons from 1 to 3.6 GeV

[Reinhard Schumacher]

December 19, 2009

S. Anefalos  Pereira, et al, “Differential cross section of gamma n --> K+ Sigma- on the bound deuteron with incident photons from 1 to 3.6 GeV”

I want to apologize in advance for waiting so long to comment on your paper:  the crush of end-of-semester tasks has kept me too busy for two weeks.  

My overall impression is that the data look good and I am very glad that CLAS is finally going to publish something useful on strangeness production off a deuteron target.  We have been promising this for too long a time.  But now you have a nice result....

I am surprised that you are trying to send this to Physical Review Letters.  They are quite likely to reject it because, apart from the experimental results themselves, there is too little that is “new” in this paper.  PRL does (or at least should) reject manuscripts that don’t either settle or raise some point of physics interest.  I think you might make some improvements if you, for example, have more than one calculation to compare with.  The Ghent model is a fine thing to compare to,  but you are unable, I suppose, to learn from the comparison to your new CLAS data what is “wrong” with their prediction for the neutron channel.  If you could do that, PRL might be more inclined to accept this paper.  On the other hand, could you get a prediction from, say, JM Laget, in order to have a second set of curves?  He could then also make some statement about what is needed to make the curves fit the data.

In the Abstract you claim to be the first “complete set” of strangeness data, etc., etc.   This phrase could be confusing to the reader since “complete set of measurements” is usually taken to mean you have decomposed the reaction mechanism down to the amplitude level.  This is obviously not the case here.  I would be happy if you just take out the word “complete”.

Page 2, column 2 near bottom, and Figure 1:  why do you make a symmetric 3 sigma cut on the proton peak?  There is no background on the low mass side, and it appears that you are throwing away about 10% of your data for no reason.  Please comment.

Page 2, column 2, last line, and Figure 2:  your choice to fit the Sigma- with a Lorentzian peak strikes me as odd, and it needs some discussion and/or further work.  There is no reason for the line shape you are using in the fitting to be Lorentzian.  It should be a regular Gaussian if things were ideal.  That is, the natural width of the Sigma- is so narrow that we cannot see it.  Hence the shape you see is determined by experimental resolution.  Experimental resolution is generally has a Gaussian profile... except when it does not!   How do you know that your yield is not better represented by a Gaussian on top of a slightly different polynomial background?  This is important, since your cross sections are directly related to the area under this yield curve.    What I would like to see is an overlay of the Monte Carlo distribution of events on top of the real data.  If you are correct in your procedure, then the Monte Carlo should look the same as the data.  You could the!
 n argue that it is OK to use a pure Lorentzian fitting function.  But if you are wrong, then your systematic error needs to be evaluated and understood.  

Figure 3:  Why are there a few seemingly isolated data points at large backward angles?  For example in the bins at W=3.05, 3.25, and 3.55 GeV, these points are hard to see and the error bars are invisible.  My suggestion:  put the bottom three rows of plots on a log scale, so the reader can more easily see the behavior of the data at large angles.  

A final comment is this: if you really do want to sent this to PRL, the paper is too long by at least half a page.  You will have to cut out a lot of paragraph, for example in the experimental discussion on page 2.  However, by then, the paper will be rather sparse.   Your better course of action might be to lengthen the paper by adding more experimental details, and then submit it as a short regular article in Physical Review C.

Awaiting the next draft,

Happy holidays,

Reinhard.