[Clascomment] OPT-IN: Measurement of the proton spin structure at long distances

[Daniel Carman]

Dear Alexandre, Xiaochao et al.,

I have read through the draft of your paper on the spin structure of the proton
at low Q2. While I am not an expert on this physics, I pass along my comments
below. If you have any questions, let me know.

Regards,
Daniel
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Page 2:
 - Line 26. Awkward. I suggest "... using a polarized electron beam to probe a
   polarized proton target at ...".
 - Line 29. Use "... i.e. at long distances.".
 - Line 31. Use "... produced by Jefferson Lab's ...".

Page 3:
 - Description of CLAS starting at line 37. I think that this will be a bit
   confusing to the reader. You refer to the CLAS NIM paper that shows a six
   sector detector, but only one sector has a modified Cherenkov detector. You
   do not make this clear for the reader. Did you only analyze the data in the
   sector with the modified Cherenkov or did you include all six sectors? The
   text seems to indicate (from the articles used) that you use only a single
   sector. Please make this clearer to avoid confusion.
 - Line 39. Use "... detector for charged particle tracking, a scintillator
   hodoscope for particle time-of-flight measurements, ...".

Page 4:
 - Line 69. Use "... [12,13], which makes it possible ...".

Page 5:
 - Fig. 2 caption. Keep units with their values so they don't split over lines as
   it appears here. In latex, if you use "... 0.016$~GeV$^2$ ...), the problem is
   removed.
 - Line 75. Use "cross section".
 - Line 80. Use "... (VVCS) amplitude that is calculable ...".

Page 6:
 - Line 87. Use "... tail (see Methods section)."

Page 7:
 - To emphasize the new low Q2 data measured with this data, highlighting the
   new data so that it is not buried under the existing published data, you might
   consider changing the aspect ratio of Figs. 3, 4, and 5 to make the plot fill
   more of the column width.

Page 10:
 - Line 147. Use "... QCD calculations to the spin-dependent ...".
 - Line 162. Use "... from JLab experiment E08-027 ...".
 - Line 164. Use "higher-order".

Page 11:
 - Line 177. Use "Uncertainties in the radiative corrections ...".

Page 12:
 - Line 210. Use "... efforts that resulted in ...".

References:
 - Please include collaboration names in the references (e.g. CLAS Collaboration) after the
   lead author.
 - Ref. [2] does not seem appropriate for lattice gauge theory. This is the PDG citation.
 - Ref. [4]. The second reference in the list is incomplete - missing volume and page number.

Supplementary Material:
 - The first table is given as Table II, but there is no preceding Table I.
 - Table II caption:
   - Line 1. Use "... and the full ranges, along with their ...".
   - Line 2. Use "Figs.~4 and 5 of ...".
 - The table in supplementary material Part II needs a caption.
 - The figure on the last page needs a caption.