[Clascomment] OPT-IN: Determination of the Proton Spin Structure Functions for 0.05 < Q2 < 5 GeV2 using CLAS

[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 a few mostly 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 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 few hints: discussion refers to the Roper P11 (please also change the S1 -> P11) 
as not prominentin 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 3 times wider than that of the Delta (and the D12(1520) and S11(1535)) .  
Also, the discussion of D13 lacks some nuance. Also the 3rd resonance lacks details. 
Amplitudes for F15 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) is 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 always 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.    
 
line 1839-1842: The "Duality" discussion in section H is rather 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 
5 GeV^2 is close to the DIS prediction. Even for the 4th resonance region the statement 
only applies to Q^2>3GeV^2. 

We should not try to put everything into the corset of "Duality". Resonances have their 
own great interest relative to QCD and there is much to be learned about strong
QCD from studying excited states of the nucleon individually. 

Volker