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

[Volker Burkert]

Dear lead authors,

I have a few comments on the eta/eta' photo production paper.

1) Introduction, page 1, left column, line 6-9. 
   The references [3-5] are utterly outdated. The most recent one is from 2002, and
   thus misses all of the recent analysis that discovered based on CLAS data up to 10 
   states, either new ones or significantly improved the evidence (start ratings). Most 
   of the new entries in ref. [2] are from the Bonn-Gatchina partial wave analysis of CLAS
   data. I have recently put out an e-print (arXiv:1610.00400) from a talk I gave at a 
   workshop in Bonn where you can find the corresponding references.  You may also 
   want to add some more text to the introduction to emphasize the importance of 
   excited baryons in the evolution of the early (microsecond old) universe. These are 
   new insights just obtained in the last couple years.  

2) Equation (5): when describing the ingredients, please don't us "was" but "is" or 
   "denotes". 

3) Systematic errors: We claim the systematic error in Sigma is 6%. Do we mean it
    is +/-0.06 ?
    If it were really 6% it would mean for a Sigma=0, the error would be 
    0 x 6% = 0. I assume this is not meant. Please correct the text.   

4) Section "Results", 2nd paragraph, and Fig. 2: 
   You gloss over the obvious discrepancy of our data with the CB-ELSA data for 
    Eg=1.475GeV. Four of the points in the backward hemisphere are systematically 
    (and increasing in magnitude for more backward angles) different from the 
    CB-ELSA data (open red circles). A referee may notice that and challenge the 
    statement that " .. the results reported here generally agree in magnitude within 
    uncertainties.) While this is correct for all other energy bins, it is obviously not 
    correct for this particular bin.You should address this head on. I assume that 
    this has been noticed by Patrick and tried to understand the origin. If this is so, 
    then the effort should be described and true conclusion justified. I also noticed 
    that in the following bins the trend of the backward points changes and asymmetries
    tend to increase at more backward angles.  Could it be that this trend sets in 
    already at 1.476 GeV, while the CB-ELSA data were measured at somewhat smaller
    energies (1.472GeV), or there is systematic uncertainty in the energy definition of 
    the polarization peak. 
    In any case, we should comment on the discrepancy.  

    On the eta' data, I think the statement "The data obtained here are consistent 
    with the GRAAL data regarding the presence of a sin^2(theta)cos(theta) dependence 
    for Sigma" is not supported by our data. We could only make such a statement 
    if a fit with this angular modulation would be done to our data and a non-zero 
    magnitude came out of that fit with high significance.  
    The best we can say is that our data do not contradict that behavior", but they also
    wouldn't contradict a result Sigma ~constant.  

    The phrase " .. perhaps consistent..."  should be eliminated. It is meaningless. 

5)  Figure 3.  The choppiness of the data in at least two energy bins (E=1516, 1676 MeV) 
     is concerning. 
    
6) Figure 2, horizontal scale. Please change the scale so that the numbers fit into 
    the allowed space. May be change the angle cosines to begin at -0.75 and 
    end at +1, or something to avoid the overlap of +1 and -1. 

7) Discussion: The observation that N(1900)3/2+ seems very important should be 
    emphasized and perhaps added to the abstract. Note, that this state has a *** rating,
    so additional evidence can help bring this is a **** status, which we should be proud 
    of. 

8) Conclusion: page 7, right column: line 6: N(1720)½+ is not a know resonance. It is
    either N(1720)3/2+ or N(1710) ½+.   
    Further down the importance of spin observables are mentioned. Sigma is not really a 
    genuine spin observable (actually it is a coherent sum of two circularly polarized photons 
    with fixed but opposite phase rotations). It is better to call it a "polarization observable". 
    When mentioning other upcoming spin observable measurements for multiple reactions,
   it may be good to refer to one or two examples. 

9) Acknowledgements: Please add the correct contract number and phrase for JSA/DOE 
    that you get from recent CLAS publications.