[Clascomment] OPT-IN: Resolving the proton form factor problem by comparing electron and positron scattering from the proton

[Daniel Carman]

Larry et al.,

I have read through the draft TPE paper dated November 14. For the most part I think the
paper is well constructed, but I have a list of suggestions to improve the readability.
Let me know if you have any questions.


					        Regards,
						
						Daniel

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Page 1.
- Institution list. Use "Santa Maria".
- Abstract.
  - Line 6. Use "liquid-hydrogen".
  - Line 8. Use "... simultaneously, as well ...".
  - Line 9. Use "systematic uncertainties".
- Missing PACS numbers.
- Line 17. Use "... GeV$^2$ [1-9]".
- Line 36. Use "... form factor extract $G_E$ from ..." (You introduced G_E as the electric form
  factor and you now call it the charge form factor. This helps to improve clarity.)
- Line 38. Use "... because the $G_E$-dependent part of the cross section is small compared to the
  $G_M$-dependent part at large $Q^2$, ...".
- Line 44. Use "... virtual photon. These corrections are therefore not ...".
- Line 52. Use "... positron beams, existing older measurements of ...".
- Line 54. Use "However, two new experiments, ...".
- Line 58. Use "In this work, we used a unique ...".

Page 2.
- Second line from top. Use "higher-order QED".
- Line 7. Use "... then used the photons ...".
- Line 9. Use "... (RL) gold foil to produce ...".
- Line 11. Use "... diverted by the Hall B tagger magnet [29] ...".
- Line 14. Use "... gold foil to produce ...".
- Line 20. Use "30-cm-long liquid-hydrogen".
- Line 22. Use "... line, see Ref. [28]".
- Line 32. Add ref. for CC (G. Adams et al., NIMA 465, 414 (2001).
- Line 35. The only system without an explanation of its purpose is the EC here. Use "... (EC) [33]
  for electron/pion separation".
- Line 35. Use "The CLAS momentum resolution ...".
- Line 42. Use "A compact mini-torus magnet placed ...".
- Line 47. Use "... uncertainties between the $e^+p$ and $e^-p$ final states due to ...".
- Line 57. Problem with opening quotation on "left beam". Also, it is not clear what you mean by "left beam".
  Please clarify.
- Line 59. Problem with opening quotation on "ratio of ratios".
- Line 65. Use "... events for the difference chicane ($c$) ...".
- Line 66. Use "... polarity settings."
- Line 84. "... thus matching the acceptances for electron and positron.". You wording is unclear to me.
  You cannot match the acceptance for electrons and positions for a given field polarity configuration
  given the different paths through the spectrometer. Please clarify what you are trying to say.
- Line 88. Use "... incident lepton beam energy ...".
- Line 90. Use "Since elastic lepton-proton ..." (to cover both e+ and e- beams).

Page 3.
- $E_l$ is not defined here.
- Add comma after $\Delta p_p$ equation.
- Line 7. Shouldn't this be "$\Delta E_l^{\pm}$?
- Line 8. What is meant by "We identified positions and electrons kinematically."?
- Fig. 1. Are the red and blue curves fits to the data? Why are you not showing histograms of the data?
- Line 27. Use "through CLAS and ...".
- Line 28. Use "... original lepton or oppositely charged .. malfunctioning counter.".
- Line 40. Use "... polarity was identical ...".
- Line 46. "The negative chicane data for magnet cycle two were discarded ... TPECal were anomalous.". This
  is a worrying statement to me that raises alarm bells. What specifically was the reason for the problem?
  If you cannot say, how can you be 100% certain that this same problem didn't affect other settings are
  a lower level and you are attributing to a TPE signature? I think that you need to consider your wording
  here with some more care.
- Line 46. "This effect also appeared ... cross section ratio." I cannot make sense of what you are trying
  to say here for certain.
- Line 49. Use "... that had a uniform acceptance ...".
- Line 56. "The acceptance correction factors are all within 0.5% of unity ...". What are you trying to say?
  Are you saying that the e+p and e-p acceptances in CLAS are 99.5%? Please clarify your point here.
- Fig. 2 caption.
  - Line 3. Use "GeV$^2$".
  - Line 4. Use "... 1.45~GeV$^2$ data.".
  - Remove the last sentence as it repeats exactly what is stated in the second sentence."
- Line 59. Use "Our TPE data ...".
- Line 60. Use "A small scattering angle $\theta$ corresponds to a virtual photon ... and a large ...".

Page 4.
- Second line from top of page. You have not said anything about your Q2 bins. What are your bin centroids 
  and extents?
- Line 4. Provide a reference for the peaking approximation.
- Line 13. Use "... vertex cuts was estimated as the difference ...".
- Line 15. Use "... cuts was estimated as the difference ...".
- Line 17. Use "... selection was estimated as the difference ...".
- Line 32. Use "... was estimated to be ...".
- Fig. 3. Are the model curves predictive or have any of the data shown (older data or this data) used to
  provide constraints? That is not apparent from what is included here.
- Fig. 3. Provide references for "previous world's data".
- Right column first line. Use "... to a linear function in ...".
- Right column second line. What do you mean by a "reduced cross section"? Is this a ratio of e+p/e-p or
  something else?
- Line 61. Problem with opening quotation on "Unpolarized + TPE cross section".
- Line 64. Use "$e^+p/e^-p$.

Page 5.
- Fig. 4 caption. You need units of GeV^2 on the Q2 denominator of G_D^2.
- Fourth line from top of page. Use "... measurements. At ...".
- Line 10. "However, those calculations are not quite large enough to resolve the discrepancy at higher
  $Q^2 [14,34]." Are you saying that at higher Q^2 that TPE effects are not sufficient to resolve the
  discrepancy between polarized and unpolarized data? Are you saying that the calculations are not fully
  trustworthy at higher Q^2? Maybe you are saying something else?
- Line 18. Use "... of TPE effects and examine ...".
- Ref. [25]. Do not include arXiv number for published work.
- Ref. [34]. Use "... Yang, arXiv:1407.2711, (2014).".
- Ref. [35]. Use "... Collaboration), arXiv:1303.2160, (2013).".
- References: "et al." should be in italics font everywhere.
- Ref. [28] should include "(CLAS Collaboration)" after et al.
- Your data should be included in the CLAS physics database and that reference should be included in this
  reference list.
- You mention in the paper about "supplementary material". What do you mean by this?