[Clascomment] OPT-IN: Photon beam asymmetry Sigma for eta and eta' photoproduction from the proton

[David Ireland]

Dear Barry et al.,

Congratulations on the very nice data.

I have just a few comments on the present draft:

- The abstract: just a couple of suggestions to strengthen the message:
"For γp → ηp, the results obtained in these studies compare favorably with the relatively few data..." 
change to 
"For γp → ηp, the results obtained in these studies improve upon the relatively few data..."

and
"For γp → η'p, the results obtained are consistent with the few previously published measurements for this observable near threshold, but greatly extend the measurements
to higher energies." 
change to 
"For γp → η'p, the results obtained greatly extend the range of measurements to higher energies, and are consistent with the few previously published measurements for this observable near threshold."
[i.e. the advance is in the new, better, more extensive results, and by the way they are also consistent with previous measurements in a more limited range.]

- Page 2, first paragraph: A complete measurement is only a mathematical concept. Some of the "complete sets" mentioned in theoretical treatments are impossible to obtain, without also obtaining additional observables. In other words experiments can only be done in combinations of beam, target and recoil polarizations. Crucially, measurements that are "complete enough" to extract amplitudes also require sufficient accuracy (see http://dx.doi.org/10.1103/2FPhysRevC.82.025204 and http://dx.doi.org/10.1016/2Fj.physletb.2016.05.069). I think it would be worthwhile mentioning this, especially as this is an experimental paper. 

- In the discussion, it might be worth mentioning the gamma+p->KY measurements that we published recently with CLAS (http://dx.doi.org/10.1103/2FPhysRevC.93.065201). These data are also being used by Bonn-Gatchina and Juelich-Bonn as part of their coupled channel analyses. It would be good to reiterate that CLAS is producing a great deal of the experimental data.

- Could you please add "the UK's Science and Technology Facilities Council" to the list of acknowledgments?

Best wishes, 

Dave