[Clascomment] OPT-IN: First-time measurement of Timelike Compton Scattering

[Daniel Carman]

Dear Pierre,

I have read through the draft of your CLAS12 paper on TCS that is intended for
publication in the Phys. Rev. Lett. I do not have substantive comments on the physics.
However, I do have some comments on grammar/style to try to improve the presentation
a bit. If you have any questions, let me know. Congratulations on getting to this
point. I appreciate how hard you have worked on this analysis. Very nice work.

Regards,
Daniel
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General:
 - In most of your figures the labels on the plots and the axes are too small.
   - Fig. 1: increase size not only of the labels but also the line thickness
   - Fig. 4: increase size of plot titles and axis labels/numbers
   - Figs. 5, 6, 7: increase size of axis labels 
 - Be consistent with how you list the final state. You have all of the following
   combinations: $e^+e^-p'$, $e^+e^-p$, $p'e^+e^-$, and $e^-e^+p$.
 - Be consistent with usage of "cross section" vs. "cross-section".

Page 1:
 - Line 29. Use "(FFs)".
 - Line 30. Use "(PDFs)".
 - Line 32. Use "... FFs and PDFs [2].".
 - Line 40. Use "... experimentally. Deeply Virtual Compton Scattering ...".
 - Line 44. Use "... process, Timeline Compton ...".
 - Line 55. Use "... applies (see Fig. 1, left). The ...".
 - Line 62. Use "... apply to the other GPDs E, \tilde{E}, and \tilde{H}."
 - Line 65. Use "... state (see Fig. 1, right).".

Page 2:
 - Line 81. Use "As presented in Refs. [15,16], the BH contribution dominates over
   the TCS contribution in the total cross section by two orders of magnitude in the
   kinematic range accessible at Jefferson Lab (JLab).".
 - Line 92. What the meaning of the "--" superscript on M? This is pretty cumbersome
   notation if it does not have any meaning.
 - Line 108. Begin a new paragraph with "In this work, two ...".
 - Fig. 2. This figure is confusing to me. What defines the plane containing the p'?
   You need two vectors to define a plane?
 - Eq. 5. What is the meaning of the "circle-dot U" subscript on the asymmetry? Again,
   this is pretty cumbersome notation if you do not explain it to the reader.
 - Line 121. Use "... and allows access to the imaginary ...".
 - Line 122. Use "Here the superscript $+/-$ stands for ...".
 - Eq. 6. You have changed notation here compared to Eq. 5. In Eq. 5 you used "sigma"
   for the cross section. In Eq. 6 you used "dsigma". Was this intentional? What does
   it mean?
 - Eq. 6. I find your notation confusing. Doesn't F mean 0 deg to 90 deg and B 90 deg
   to 180 deg? For the different F/B terms you are integrating over these angular regions.
   Your notation in Eq. 6 looks like you are referring to fixed angles and not an
   integration.
 - Line 131. Use "... coverage of the detectors, compared ...".
 - Line 141. This is the first time that you have mentioned that your photons are not
   real but quasi-real. It seems to me that you should introduce this notion earlier
   to explain the concept to the reader.
 - Line 143. Use "... one positron, and one proton.".
 - Line 158. Use "... = v/c$, where $v$ is ...".

Page 3:
 - Line 169. Use "... to ensure kinematics in the quasi-real ...".
 - Line 171. Use "... photon were determined ...".
 - Line 173. Use "... of the final state proton ...".
 - Line 175. Use "... electron were constrained ...".
 - Line 190. Use "... $\phi$, and $J/\psi$ ...".
 - Line 193. Use "Bethe-Heitler".
 - Line 194. Use "normalized".
 - Line 207. Use "{\it e.g.}"
 - Line 208. Begin a new paragraph beginning with "The photon polarization ...".
 - Line 210. Use "statistical uncertainties.".
 - Line 218. Use "... polarization $P_{trans}$ that can be calculated analytically ...".
   Note I also suggest to use $P_{trans}$ and not $P_{trans.}$ here and in Eq. 9.
 - Line 245. Use "... shift was calculated ...".
 - Line 247. Use "This procedure was necessary ...".
 - Line 251. Use "... associated with the binning ...".
 - Line 252. Use "... and with the rejection of ...".
 - Line 260. Use "... than the statistical uncertainties, typically ...".

Page 4:
 - Fig. 4 caption. Line 2. Use "$t$-bins".
 - Line 272. Use "$t$-dependence".
 - Line 274. Use "$t$-dependence".
 - Line 276. Use "... calculations were performed ...".
 - Line 281. Use "... models, while BH-only ...".
 - Line 298. Use "... was computed for each $-t$ bin as:".
 - Line 299. Add a comma after Eq. 10 for proper punctuation.
 - Line 300. Use "... in the forward/backward angular bins, ...".
 - Line 310. Use "... systematic uncertainties by computing ...".

Page 5:
 - Fig. 6. You mention the solid curve in the caption. What about the other curves?
 - Line 344. Use "... parameterizations that appears ...".
 - Line 358. Use "Timeline Compton Scattering".
 - Line 360. Use "... forward/backward asymmetries ...".
 - Line 361. Use "... non-zero, providing strong ...".
 - Line 379. Use "... ultra-peripheral collisions ...".

Refs:
 - Use {\it et al.} throughout.
 - [18]. This is the only reference where you are listed authors last name first and then
   initial. Be consistent with the style of your other references.
 - [19]. Use "Phys. Rev. D".
 - [20]. Use "Nucl. Phys.".
 - [40]. Use "Phys. Lett. B".
 - [43]. Use "Eur. Phys. J. C".
 - [49]. You should not list a bunch of names and then follow it with an "et al.". The purpose
   of "et al." is to replace the bunch of names.