OPT-IN: Measurement of the helicity asymmetry E for the vec(γ)vec(p) → pπ0 reaction in the resonance region

[Daniel Carman]

Dear Nick et al.,

I have read through the draft of your paper on the helicity asymmetry E in neutral pion
production with the FROST target. I found the paper well written and this helps to focus
the reader on the very impressive analysis results and outcome. I include my comments
below. If you have any questions, let me know.

Regards,
Daniel
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General:
 - I noticed some verb tense issues throughout where you use present tense instead of
   past tense. I have tried to capture all of these in my comments below.

Page 1:
 - Line 14. Abstract. Use "... Acceptance Spectrometer ...". 
 - Line 45. Use "... to the $\pi N$ decay channel ...". 
 - Line 53. Use "CLAS Collaboration". 
 - Line 56. Use "... similar analysis procedures". 
 - Line 63. Use "... polarized target ...". 
 - Line 67. Use "... extraction of the double-polarization observables ...". 
 - Last paragraph. Line 3. Use ".. of cross sections for helicity ...".

Page 2:
 - Line 76. Use "... 1/2 and -1/2, respectively;".
 - Line 78. Use "... and -1/2, respectively.".
 - Line 95. Use "... quantities that provide information ...".
 - Line 101. Use "... of experimental uncertainties.".
 - Line 116. Use "... and a new partial wave analysis (PWA), where we compare ...".
 - Fig. 1. I do not find Fig. 1 to be value added to this paper. It is so elementary
   that it seems out of place and the reference to it on line 144 is not appropriate.
   If you do decide to keep it, it should be used as a reference after you introduce
   the pi-zero CM angle in line 217.
 - Line 163. Use "... ratio of photon to electron energy ...".

Page 3:
 - Line 171. Citing Ref. [10] is not needed here as the use in line 176 is sufficient.
 - Line 173. Use "... called Dynamics Nuclear ...".
 - Line 193. Use "... built up downstream of the target), ...".
 - Line 200. Give the actual section here (don't just say "a later section").
 - Lines 210-212. The discussion of coordinate systems is not so clear. As the cross section
   is in the CM system, the axes noted here as "x", "y", and "z" thus are not lab angles. How
   are they defined?
 - Line 217. Use "(the center-of-mass ($c.m.$) polar angle of the meson in the final state)."
 - Line 217. Here you write the c.m. angle of the pion as "\theta", but in Fig. 1 you introduce
   a different notation. You need to be consistent throughout the paper.
 - Line 245. Use "... where the detector and experimental effects listed ...".
 - Line 253. Give the actual section here (don't just say "a later section").
 - Footnote #2. Use "{\it i.e.}".

Page 4:
 - Line 302. Use "... the polarization of events that ...". 
 - Line 303. Use "... that originated from the ...". 
 - Line 307. Use "... polarization was based on the ...". 
 - Line 314. Use "... (butanol) were accounted for ...". 
 - Eq.(8). Swap the order of terms in the denominator to be consistent with other usage.
 - Line 323. Use "... factor was determined ...". 
 - Line 327. Use "... we exploited the fact ...". 
 - Line 339. Use "... factor was then determined ...". 
 - Line 344. Use "... were utilized to ...". 

Page 5:
 - Fig. 2.
   - The text fonts are too small to be readable.
   - The vertical black lines are too faint to be read.
 - Fig. 2 caption.
   - Line 1. Use "... mass squared distribution ...".
   - Line 2. Use "Two PDFs were used to determine the ...".
 - Line 348. Use "... was determined using ...".
 - Line 353. Use "... that enabled us to ...".
 - Line 354. Use "... that originated within ...".
 - Line 355. Use "... that was dependent ...".
 - Line 363. Use "... were determined using ...".
 - Line 369. Use "... its uncertainty of the fit to the butanol ...".
 - Line 378. Spurious space between "$\sim$" and "4\%".
 - Line 379. Use "... PDFs had on describing ...".
 - Table I. The listing of the total point-by-point (absolute) systematic is confusing.
   Better to give the range "0.023 - 0.301" instead of using "$> 0.023".
 - Table I caption. Line 2. Use "... determination of the double-polarization observable ...".
 - Line 388. Use "A summary of the systematic uncertainties ...".
 - Line 389. Use "... are split into a ...".
 - Line 390. Use "... that was applied ...".
 - Line 392. Use "... that was applied as a ...".
 - Line 397. Use "... 2.23~GeV for a side ...".
 - Line 406. Use "{\it i.e.}".
 - Line 410. Use "(green dashed line)".
 - Footnote #3.
   - Line 1. Use "... uncertainty was added to ...".
   - Line 2. Use "... and was treated independently ...".

Page 6:
 - Fig. 3 caption.
   - Line 5. Use "... are indicated by the shaded ...".
   - Line 6. Use "... at the bottom of each plot.".
   - Line 8. Use "PIONMAID-2021". (or whatever the standard is, but be consistent
     throughout the paper).
   - Line 10. Use "... respectively. The CBELSA data were included in the SAID, MAID,
     and Bonn-Gatchina fits.".
 - Line 427. Use "(green dashed line)".
 - Line 435. Use "energy-dependent".
 - Line 436. Use "$K$-matrix".
 - Line 438. Use "... Ref. [38], was used in ...".
 - Line 443. Use "$K-matrix [39] ... $\sigma$ are indices that label the channels, ...".
 - Second line after Eq.(10). Use "... channels that appear ...".
 - Line 457. Use "... in the fits of the photoproduction ...".
 - Line 460. Use "$K-matrix".
 - Line 465. Use "... from SAID to the present ...".
 - Line 473. Use "... by a Breit-Wigner shape, ...".
 - Line 481. "... with multi-particle final states." This is a meaningless description as all
   reaction have multi-particle final states. Can you better clarify?
 - Line 484. Use "energy-dependent".

Page 7:
 - Fig. 4 is very confusing in the ordering of the subplots. I strongly suggest you remake
   this figure and put the plots from upper left to lower right in order of increasing W.
 - Fig. 4 caption.
   - Line 1. Remove comma after "Double-polarization observables".
   - Line 5. Use "... the bottom of each plot.".
 - Line 489. Use "... in the final state, as well as photon-induced reactions ...". 
 - Line 494. Use "... of the fits obtained ...".
 - Table II caption.
   - Line 1. Use "... $\chi^2$/data point for the new ...".
   - Line 3. Use "PIONMAID-2021".
 - Fig. 5 caption.
   - Line 3. Use "vs.". 
   - Line 6. Use "The new CLAS FROST ...". 
   - Line 7. Use "... are included in the fits.". 

Page 8:
 - Fig. 6 y-axis label. Use "$\chi^2$/data point".
 - Fig. 6 caption. Line 4. Use "... double-polarization observable ...".
 - Line 504. Use "... with the $N(2120)3/2^-$".
 - Line 506. Use "... associated with the $\Delta(1940)3/2^-$. These states ...".
 - Line 517. Use "...double-polarization data above photon energies of 2~GeV, ...".
 - Line 520. Use "... Bonn, and LEPS ...".
 - Line 521. Use "... higher precisions and broader angular ranges, but ...".
 - Line 523. I do not know what you are trying to say by "or truncated partial-wave
   analysis" and how this is connected to a "complete experiment".
 - Line 535. Use "double-polarization".
 - Line 543. Use "... inclusion of this newly ...".
 - Line 546. Use "... with the $N(2120)3/2^-$".
 - Line 547. Use "... with the $\Delta(1940)3/2^-$, while ...".
 - Line 550. Use "... the impact of this new dataset and their findings will be ...".

References:
- [8]. Use your fancy notation for "E" in the title.
- [13]. Reference incomplete. Use "R. Walker, "Phenomenological Analysis of Single Pion
  Production", Phys. Rev. 182, 1729 (1969)."
- [23]. The CLAS paper is not a CLAS Collaboration paper.
- [32], [33]. Use your fancy notation for "E" in the title.
- [34]. Use your fancy notation for "T" in the title.
- [35]. Use your fancy notation for "F", "P", and "H" in the title. Also use "$N^*$" not
  "N$^\ast$".
- [38]. Use "$\pi N$".
- [39]. Use "$S$-wave", "$\pi$", and "$\eta$".
- [41]. Use your fancy notation for "T" in the title. Use "$\pi N$".
- [46]. Use "$D$-matrix".