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

[Volker Burkert]

Dear lead authors,

 This is a long awaited important paper that is well written and deserves to be 
published as soon as possible.  I have only very few minor comments.  

line 127: you define Mott cross section as 1/2sigma_M = 4E'.../Q^2,
  why not simply write: sigma_Mott = 8E'.../Q^2? 

line 194-195: I suggest include "...dominance expected from QUARK MODELS and from pQCD". 
The latter only "predicts" the asymptotic behavior, no transition from A1=-1 to A1 = +1 is predicted,
while several quark models predict the transition quite well. 

line 825: "Moller polarimetry requires a magnetized iron target.."  Later you give the correct description 
 as "Permendur (49%Fe, 49% Co, 2% Va). So, it doesn't have to be iron. I fact pure iron gives lower
 polarization at the 120G external magnetic field. So, you should replace "iron" with
 "highly magnetizable material (foil)" or something similar.  

line 545: The polarized target is described in great detail. However, I was missing a reference to our 
NIM paper on the polarized target,  C.D. Keith et al., NIM A501 (2003), 327-339. 

line 623: You refer to ND3 material as behaving differently from NH3. While this is true, it is the 
only place I found where ND3 is even mentioned. I think it is misplayed here and should be deleted. 

line 1465 - 1499: The discussion of the various resonance contributions is not up to our current 
knowledge. It should be re-written by also consulting (and referencing) several CLAS papers.
 1)  I.G. Aznauryan et al. (CLAS), PRC 80 (2009) 055203 
 2)  K. Park et al. (CLAS), Phys.Rev.C91 (2015) 045203  and the review paper
 3)  I.G. Aznauryan, V.D. Burkert, Progress in Particle and Nuclear Physics 67 (2012) 1-54. 

A couple hints: discussion refers to the Roper P11 (please also change the S1 -> P11) as not prominent
in electroproduction. While this is true at low Q^2, where the leading amplitude crosses 0, it is not true 
at Q^2 > 1.5GeV^2. In fact, the Roper transverse amplitude is larger than the Delta(1232) at 
high enough Q^2 (>2GeV^2). The reason that it doesn't show up as a strong "peak" is that its width
 is about 2-3 times wider than that of the Delta (and the D12(1520) and S11(1535)) .  
Also, the discussion of D13 lacks some important details, as well as the 3rd resonance. Amplitudes 
for F15(1680) have been extracted from the CLAS data and are included in one of the papers above (2).  
For the 4th resonance region you seem to imply, that because of their higher spin the 3/2 
amplitude would dominate. While this is the case for Delta(1232) it is not necessarily so for higher 
states, and in particular, as you state earlier, pQCD (and in some cases quark models) tell us that 
the helicity-½ amplitudes should dominate at high enough Q^2. Unfortunately, not much 
is known about their electro-coupling amplitudes.

Notation: The now accepted notation for baryon resonances is N(mass)spin^parity or 
Delta(mass)spin^parity. This is the notation of resonances used by the PDG. I suggest to 
use those as well.    
 
line 1839-1842: The "Duality" discussion in section H is well balanced. However, the conclusions 
seem to be disconnected from the discussion. The statement : Duality applies both to individual 
resonances (except Delta), and to the resonance region as a whole .. above Q^2~2GeV^2. 
While the latter part is somewhat supported by the data, the former is clearly not. The data 
in Fig. 40 clearly show large discrepancies between data and DIS behavior for the first AND 2nd 
resonance regions. At a Q^2<4.5GeV^2 there is a 20-30% difference for nearly all data points. 
Only the highest point at 5GeV^2 is close to the DIS prediction. Even for the 4th resonance 
region the statement only applies to Q^2>3GeV^2. 

Personal comment: I don't understand why many people try to put everything into the corset 
of "Duality". The difference is what makes resonance interesting to understand within QCD. 
Resonances have their own great interest relative to strong QCD and need to be studies individually. 

Volker