OPT-IN: Measurement of Beam-Recoil Observables Cx and Cz for K+Λ Photoproduction

[Reinhard Schumacher]

I read your new paper on Cx and Cz in Lambda photoproduction today (while I am traveling on vacation).    Congratulations on getting this new result to this stage.    Of course I am glad to see that the results are in agreement with the Bradford et al results that we published from CLAS in 2007.    The state of much of the theoretical modeling remains pretty poor, I see.  I have a couple of comments that you can consider.

- The Bonn-Gatchina model is not used or referred to in this paper.    I think I know why:   getting their help to use their model together with these new results would add another layer of complexity to organize getting this result published.   That is probably OK.   But on the other hand, the paper has no scholarly paragraph that outlines the full range of work that was done with the 2007 results.    You probably want to add that in order to avoid a referee dinging you on that omission.   That is, you want to mention all of the places where the previous results have already been studied and cite the most important ones;  there have been almost 150 citations!   I mention the BG model in particular because I think their fits were most successful so far.    (I don’t have the papers at my fingertips here on my trip.)

- The draft paper does not discuss the very interesting question of the quantity “R” that gets introduced and evaluated in the 2007 paper.   You are leaving some low-hanging fruit to hang.    The idea is that the three components of the Lambda polarization seem to add up quite close to unity.    We showed this for the first time in the 2007 paper, and as far as I have seen it is still not something that is explained in a qualitatively convincing way.    That is, various models (eg BG) fit the data moderately well quantitatively, but they don’t “explain” in non-mathematical terms why the Lambda should be fully polarized in this reaction.   It seems to me that you should do the (quite small) amount of work to compute this quantity from your new data when it is combined with other CLAS results for the out-of-plane polarization P.   Nobody is going to do it for you.   It will increase the impact of the paper.

- Line 645: you claim that the Lambda is created “fully polarized” because Cz is close to unity over some range of the reaction.    This is not a correct statement.    Any reasonable understanding of that phrase means that the vector sum of the three components Cx, Cz, and P should be close to unity.   This relates to my previous point about computing R.    I think you should do that calculation and present it.   Otherwise CLAS will not retain intellectual “ownership” of that physics result.

- Lastly, it looks like you have a perfectly good set of data to also extract these observables on the Sigma0.   Do you want to say anything about why you did not include that set of observables as well?  Maybe I missed it in the paper, but if it is not there it deserves a sentence or two somewhere.

Let me know if I can help in some way here,

Best,
Reinhard