[Clascomment] OPT-IN: Measurement of deeply virtual Compton scattering off Helium-4 with CLAS at Jefferson Lab

[Daniel Carman]

Dear Raphael et al.,

I have read through the draft of your DVCS paper on 4-Helium dated Jan. 11, 2021.
This is a very nice analysis and the presentation given is quite clear. My comments
are mainly grammatical and the draft could use some improvements on this front. If
you have any questions, let me know. Good luck with the rest of the process and the
submission to the journal.

Regards,
Daniel

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General:
- You have written parts of the paper in the present verb tense. This is not standard
and makes the paper read awkwardly. Most of your verbs should be in the past tense
and I indicate this below.
- You are not consistent with your speed of light units throughout. I suggest that you
let c=1, which also simplifies the notation.
- Be consistent throughout with your notation of "beam spin asymmetry" vs. "beam-spin
asymmetry".
- The words in the section headings should be capitalized.
- Be consistent with your spelling of the color "gray" vs. "grey".
- You should use $^4$He as opposed to $^4He$ for proper notation.
- According to the CLAS Collaboration policies, all data should be included in the CLAS
  Physics Database (see http://clasweb.jlab.org/physicsdb/) with an appropriate reference
  given in the paper. For questions about format and entering the data, you can talk to
  Victor Mokeev.

Page 1:
 - Line 16. Use "(GPDs)".
 - Line 17. Use "(TMDs)".
 - Line 25. Use "(FSIs)".
 - Line 27. Use "... GPD is involved in the process ...".
 - Line 28. Use "HERMES Collaboration".
 - Line 30. Use "CLAS Collaboration".

Page 2:
 - Line 35. Use "... [6,7,8,9], and we summarize here only the ...".
 - Line 37. Use "... are the incoming and outgoing quark momenta, ...".
 - Line 38. Use "... \Delta^2$ is the squared transferred four momentum to ...".
 - Line 42. Use "... spin $J$, therefore for a spin-0 hadron like the helium-4 ...".
 - Line 43. Use "... two GPDs. For a spin-1/2 hadron like the proton, ...".
 - Line 44. Use "... flip, and they are called transversity ...".
 - Line 47. Use "... full phase space of the parameters.".
 - Line 48. Use "... simplify the notation, we define the ...".

Page 3:
 - Line 60. Use "The interference terms significantly increase the ...".
 - Line 61. Use ",,, asymmetries (BSAs), which are ...".
 - Line 68. Use primes on the scattered electron and 4He.
 - Line 70. Use "... measurement of the BSAs noted ...".
 - Line 70. Do not hyphenate (as a general rule) "-ly" words. So, use "longitudinally
    polarized".
 - Eq. 5. End equation with a comma for proper punctuation.
 - Eq. 5. $x_A$ is not defined here but on line 84. I suggest that you move the definition
   to just after this equation for clarity.
 - Line 75. Use "... for a spin-zero target, respectively.".
 - Line 77. You are not consistent with your spelling of the author. Here you use
    M\"uller (and on line 495 as well) and in Ref. [13] you use Mueller.

Page 4:
 - Fig. 3. The horizontal line labeled "Factorization" is not explained or mentioned in
   the text.
 - Eq. 6. End equation with a comma for proper punctuation.
 - Line 79. Use "... real parts, respectively, of the CFF ...".
 - Eq. 9. Use "\cos" to make cos appear in rm font.
 - Line 83. Use "... leptonic tensor [14]. Their explicit expression ...".
 - Line 85. Drop "The" at the beginning of this sentence.
 - Line 91. Use "Recent work indicates that the ...".

Page 5:
 - Fig. 4 caption. Use "Eqs. 7 to 10.".
 - Line 99. Use "... exact kinematics of the reaction.".
 - Line 100. Use "Moreover, in general, the nucleon ...".
 - Line 101. Use "... DVCS reaction and the remnants of ...".
 - Line 102. Use "The latter lead to ... channels, in particular, ...".
 - Line 103. Use "... such background reactions.".
 - Line 104. Use "... exclusivity constraint ...".
 - Line 106. Use "... motion and final state interactions possible.".
 - Line 108. Use "... effects in deep inelastic scattering (DIS) [16] and quasi-elastic
   scattering [17], and we can ...".

Page 6:
 - Fig 6 caption. Use "The $\sin \phi$ moment ... [10]. The gray bands ...".
 - Line 112. Use "... two directly providing ...".
 - Line 113. Use "... fragments significantly limit ...".
 - Line 115. Use "... DVCS was performed by the HERMES Collaboration [10].".
 - Line 117. Use "... reproduced in Figs. 6 and 7, suffer ...".
 - Line 121. Use "... has been raised in Ref. [18] to explain ...".

Page 7:
 - Fig 7 caption. Use "The $\sin \phi$ moment ... ".
 - Line 131. Use "... that can be obtained from it.".
 - Line 136. Use "... had for its main objectives to explore ...".
 - Line 144. Use "The CLAS spectrometer was installed in Hall~B of Jefferson Lab (JLab).
   This detector was specifically ...".
 - Line 146. Use "... with multi-arm spectrometers.".

Page 8:
 - Line 148. Use "... separated by the coils of a toroidal magnet, ...".
 - Line 152. Use "... time-of-flight measurements [21]".
 - Line 153. Use "... 8 to 45 degrees, ...".
 - Line 157. Use "... to December 2009 at an electron beam energy of 6.064~GeV, with the
   beam intensity varying between ...".
 - Line 158. Use units for luminosity of "cm$^{-2}$s$^{-1}$".
 - Line 159. Use "The CLAS Collaboration".
 - Line 163. Use "... composed of 424 lead ...".
 - Line 166. Use "... in Fig. 9, is a homogeneous ...".
 - Line 170. Use "... 5~T solenoid was placed ...".
 - Line 171. Use "... electrons) curled around the beamline and never made it ...".
 - Line 172. Use "... illustrated by the simulation results ...".
 - Line 173. Use "... for low rate processes like DVCS.".

Page 10:
 - Line 177. Use "... from the BONUS setup ... GEM-based RTPC [27] ...".
 - Line 179. Use "... than toward the endcaps, ...".
 - Line 182. Use "... elsewhere [29]. Here a summary of the key elements is provided.".
 - Line 186. Use "... bag was placed downstream ...".
 - Line 187. Use "... in Fig. 11. Here we list ...".
 - Line 192. Use "beamline".
 - Line 193. Use "... to separate the gas regions.".
 - Line 195. Use "... in an 80/20 proportion.".
 - Line 202. Use "beamline".
 - Line 204. Use "... the kinematics of the helium ... electron kinematics and directly ...".
 - Line 207. Use "... available in Ref. [29].".
 - Line 210. Use "The electrons were detected with the baseline CLAS detectors. The drift
   chambers measured the kinematics of the electron ...".
 - Line 212. Use "... provided the identification. A signal of good quality was also ...".
 - Line 213. Use "... served as a time reference for all detectors. Protons were detected ...".
 - Line 214. Use "... the drift chambers measured the kinematics of ... system ensured ...".

Page 11:
 - Line 216. Use "... where their efficiency is rapidly ...".
 - Line 218. Use "... calibration, which are at the subpercent ...".
 - Line 221. Use "... cuts were used in this detector as large energy deposition was ...".
 - Line 222. Use "... which could not be separated ... at large angles in CLAS ...".
 - Line 223. Use "... highly suppressed the number ... in the calorimeter; moreover, ...".
 - Line 224. Use "Left-over accidentals were accounted ...".
 - Line 229. Use "The helium-4 nuclei were detected ...".
 - Line 232. Use "Finally, we selected events ...".
 - Line 233. Use "We applied a selection ... they originated from ...".
 - Line 235. Use "... we selected ...". Note also here you have a quantity and its units
   separated across line. Prevent this using e.g. 1~GeV$^2$.
 - Line 236. Use "... the recoil $^4$He was bound ...".
 - Line 237. Use "... energy-momentum conservation:".
 - Line 238. Use "... DVCS, we used a similar ...".

Page 12:
 - Line 242. Use "... particles were detected ...".
 - Line 243. Use "... this method was not sufficient.".
 - Line 246. Use "... we constrained the selection ...".
 - Line 247. Use "... as follows for coherent DVCS ...".
 - Line 258. Use "In the analysis, we applied selection ...".
 - Line 261. Use "... distributions only a few minor ... the distributions have some ...".
 - Line 262. Use "... resolution, which impact some ... non-linearly.".
 - Line 263. Use "... differences: wider distributions overall ...".
 - Line 264. Use "The wider distributions ...".
 - Line 266. Use "... distributions are caused ...".
 - Line 270. Use "The main signal contamination comes from the exclusive ...".
 - Line 273. Use "... we measured the exclusive ...".
 - Line 276. Use "in Figs. 14 and 15, respectively. Using this sample, we developed an ...".
 - Line 277. Use "... of the detectors. The result of this ...".
 - Line 278. Use "... of Figs. 14 and 15.".
 - Line 279. Use "... we estimated the number ...".

Pages 13 and 14:
 - Fig. 12 and Fig. 13 captions:
   - Line 2. Use "... in gray) ...".
   - Line 4. Use "... missing $P_x$ and $P_y$ components, in order left ...".

Page 15:
 - Line 282. Use "This number was then subtracted ...".
 - Line 292. Use "... we inverted the vertex ...".
 - Line 293. Use "... and requested that ...".
 - Line 294. Use "We found that 4.1\% ... samples were accidentals.".
 - Line 296. Use "To further evaluate the systematic uncertainty of the measurements, ...".
 - Line 299. Use "... beam polarization was estimated ...".
 - Line 299. I don't think your argument here is correct. The beam polarization can
   vary throughout a run period due to changes in the beam orbit, changes at the injector,
   slit changes that affect bleed-through from other halls. The variation of the Moller
   measurements vs. time is not necessary purely statistical in nature. Note also that their
   is an overall 3% systematic scale uncertainty with the Moller measurement that arise from
   the uncertainty in the polarization of the iron target foil. Please revisit your description
   here. I would recommend that you talk to Stepan Stepanyan about your final wording.
 - Line 303. Use "... an uncertainty of their expected value.".
 - Line 305. Use "... systematic uncertainty presented in the results.".

Page 17:
 - Fig. 16 caption. Use "... over the kinematic variables ...".

Page 18:
 - Line 313. Use "... as is illustrated in Fig. 17.".
 - Line 314. Use "... the systematic uncertainty associated ...".
 - Line 315. Use "... associated with the DVCS cuts, ... systematic uncertainty based on ...".
 - Line 319. Use "... almost double that measured for the free ...".
 - Line 321. Use "... first time, and we can conclude ...".
 - Line 325. Use "... in Fig. 19. The model compared to ...".
 - Line 328. Use "non-diagonal".
 - Line 329. Use "AV18".
 - Line 331. Use "... precise kinematics of each of the points in presented in Appendix B.".
 - Line 333. Use "... structure that is well reproduced ...".
 - Line 334. Use "One of the motivations for the ...".
 - Line 335. Use "... from the data. To perform ...".
 - Line 336. Use "We present in Fig. 20 the extracted real and ... of the helium-4 nucleus.".
 - Line 337. Use "... rather encouraging. The two parts ...".
 - Line 338. Use "... the need for any model assumption.".

Page 19:
 - Fig. 18 caption. Use "... middle panels) and $-t$ (lower panels). The error bars are
   statistical and the gray bands ...".
 - Center Fig. 19 on the page.
 - Fig. 19 caption. Use "... are shown by the red full line and the dashed blue line, while
   the ... magenta long dashed line.".

Page 20:
 - Fig. 20 caption. Use "... the blue long dashed line ...".
 - Line 345. Use "... difference between the nuclear models.".
 - Line 353. Use "... the relevance of coherent nuclear ...".
 - Line 357. Use "... similar to those observed with the free ...".

Page 21:
 - Fig. 21 caption. Use "... middle panels) and $-t$ (lower panels). The error bars are
   statistical and the gray bands ...".

Page 22:
 - Fig. 22 caption. Use "... are shown by the red full line and the dashed blue line, while
   the ... magenta long dashed line.".
 - Line 368. Use "Such a ratio is ...".
 - Line 375. Use "30\%".

Page 23:
 - Fig. 23 caption. Use "... are shown by the red full line and the dashed blue line, ...
   magenta long dashed line ... and the black dot-dashed line ...". (Note spurious "the"
   here.)

Page 24:
 - Line 391. Use "... the incoherent data.".

References:
 - You are not maintaining a consistent style from reference to reference.
 - Use "A. Author1 {\it et al.}, Journal {\bf Volume #}, page (year).". Include spaces
   between the different elements (in many of your references you have no spaces and
   everything runs together.
 - For some entries you include the paper title, for others you do not.
 - Please list collaboration names, e.g. "(CLAS Collaboration)", in the citation.
 - Do not give a page range, just the first page number is sufficient.

Page 26:
 - Appendix A. I am not qualified to check your equations for correctness. Hopefully
   somebody checked this all carefully. However, I pass along what I noted in my reading.
 - Line 494. Use "... and Eqs. 7 to 10.".
 - On the same line as Eq. 19 there is some spurious text "pdflatexprc - dvcshelium.tex".
 - Add a comma after Eq. 21 for proper punctuation.
 - Line 497. Use "... form factor of $^4$He.".
 - Add a period after Eq. 22 for proper punctuation.
 - Add a period after Eq. 28 for proper punctuation.
 - Eq. 28. The last term x_A t/Q^2 has an asterisk between the x_A and the t. The asterisk
   should not be there.