[Clascomment] OPT-IN:Precision measurement of g1 of the proton and the deuteron with 6 GeV electrons

[Volker Burkert]

Hi Yelena, Peter and Nicholas;  
This is a very nice paper representing an enormous amount of of work over many years. While 
some of these has been discussed in the eg1-dvcs working already I like to make a few additional
remarks. 

1) Figures: many of the figures are of substandard quality (fuzziness) and look like they came from cut & paste
from another file. For example: Fig. 6 (also replace NH_2 => NH_3, and ND_2 => ND_3),  Fig. 9, here the 
light color of the eg1b data should be replaced with a different color if we want to show the eg1b data. 
Since we don't show the eg1b from ND3 why not also drop the NH3 data?  Fig. 10, 11, 12, 13 are also fuzzy.  

2) Throughout the paper replace "error" with "uncertainties". 

3) Section D Physics results. 
I like to expand a little on the "resonance structure" discussion. I suggest to replace the text from 
"The strong peak near 1.5 GeV ......  at high as 4 GeV^2." with the following text or something similar: 

" The strong peak near 1.5 GeV seen in the xg1 plots is dominated by transitions to two strong nucleon 
resonances, the N(1520)3/2^- and the N(1535)1/2^- states. The latter is a spin=1/2 resonance
and has only helicity =1/2 transition amplitudes, i.e. A=-1. The former is a spin = 3/2 state with both 
helicity 1/2 and 3/2 amplitudes contributing to the transition, and thus can take on a range from 
A_1=-1 for \sigma_{3/2} dominance to A_1=+1 for \sigma_{1/2} dominance. As was found in the analysis of single 
pion and single eta exclusive cross section measurements from other CLAS experiments~\cite{azn1,azn2), the 
helicity structure of that state changes rapidly from A_1=-1 at Q^2=0 to A=+1 at Q^2 > 1 GeV^2. 
In the Q^2 range covered by this data set both states have A_1 =+1 to create the strong peak at 1.5 GeV.   

The peak at 1.7 GeV has a similar origin, while more resonance contribute in this mass range the 
N(1680)5/2^+ resonance was found as the by far dominant state for a proton target. The helicity structure
is similar as for the N(1520)3/2^- discussed above, and shows a dominant helicity 3/2 transition at 
Q^2=0, and also chances its helicity structure for Q^2> 1GeV^2 to helicity 1/2 dominance resulting in 
an asymmetry A_1 > 0 that is growing with Q^2. 

\bibitem{azn1} I.G. Aznauryan et al. (CLAS collaboration),  Phys. Rev. C80 (2009), 055203.
\bibitem{azn2} I.G. Aznauryan and V.D. Burkert, Prog. Part. Nucl. Phys. 67 (2012), 1. 

I also suggest to replace part of the following paragraph " We speculate ..... from this same experiment".
 (Note there is a mistake: the S31(1900) resonance is a spin 1/2 state and not a spin 3/2 state, 
and thus would produce A_1>0, not a dip. Also, spin 3/2 resonances do not necessarily produce  
a negative asymmetry, see discussion of N(1520) above.)

Replacement: "The dip could indicate significant contributions from resonances with dominant helicity=3/2 
transitions. There are a number of states in this mass range that could contribute to the observed structure. 
To investigate this possibility further, we are in the process of analyzing exclusive single pion and 
eta electroproduction from the same experiment. The analysis of pion production on neutrons may
provide information on the isospin of the resonance contribution.       

4) Please make sure the data is logged into the CLAS Physics Data Base, and give reference to it.  

5) Acknowledgements: Please eliminate the "the Emmy Noether grant from the 
Deutsche Forschungsgemeinschaft". This is a leftover from the days when Ulrike Thoma was at JLab. 
However, add the UK Funding agency. 

Volker