[Clascomment] OPT-IN:Photoproduction of two charged pions off protons in the resonance region

[Daniel Carman]

Dear Volker et al.,

I have read through the draft of your new paper on higher-level analysis of the existing CLAS 2pi
photoproduction data. I detail my comments below. Mainly they are focused on improving the
grammar/style. If you have any questions, let me know. Good luck with the submission process.

Regards,
Daniel
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General:
 - You are not consistent with your usage of speed of light units. You consistently use GeV/c for
   momentum but GeV2 for Q2. It seems better to let c=1 throughout for a consistent notation.
 - You are not consistent with your notation for baryons. Sometimes you put the charge before the
   mass and sometimes after, e.g. $\Delta^0(1232)$ vs. $\Delta(1232)^0$. Be consistent throughout.
   I would also expect the notation for mesons and baryons should use the same convention, e.g. you
   use $\rho^0(770)$ throughout.
   
Page 1:
 - Abstract:
   - Line 1. Use "Photoproduction of charged pion pairs ...".
   - Line 2. Use "... the final state hadrons ...".
   - Line 6. The use of the word "frequency" is a bit confusing. You are not talking here about decay
     rate, but about branching fraction.
 - Line 21. Use "... can dissociate the proton into a $\Delta^{++}(1232)$ and a $\pi^-$ meson ...".
 - Line 28. Use "The mass and angular ...".
 - Line 34. Use "database".
 - Line 36. Use "event-by-event".
 - Line 40. Use "... pion, photo-, and electroproduction ...".
 - Line 43. Use "Pion-induced".
 - Line 59. Use "... $Nf_0(500)$, and $N\rho(770)$ isobar ...".

Page 2:
 - Line 108. Use "... observable $E$, and ...".
 - Line 123. Use "... 1.7~GeV for $\gamma p \to p 2\pi^0$ [44] and ...". (Reposition reference location.)
 - Line 133. Use "... $\Delta(1232)^{++}$, and ...".
 - Line 147. Use "de-excite".
 - Line 162. Use "... in Ref.~[55].".
 - Line 163. Use "... were identified in Ref.~[4].".
 - Line 166. Use "... and $N\rho(770)$, which were ...".
 - Line 177. Use "... with $p$, \pi^+$, or $\pi^-$.".
 - Line 178. Use "coupled-channel".
 - Line 181. Use "$\Delta^{++}(1232)\pi^-$". (Latex problem with charge superscript.)
 - Line 182. Use "... differential cross sections as a function of squared ...".
 - Line 199. Use "... 4.019~GeV energy. The beam current ...".

Page 3:
 - Line 220. Use "... Ref.~[67].". (End sentence with a period.)
 - Line 224. Use "... density of H$_2$, 0.0718~g/cm$^3$, was nearly ...".
 - Fig. 1 caption. Line 1. Ref. [69] is not appropriate here for the source of this figure. Use only Ref. [63].
 - Line 246. Ref. [70] is not appropriate for the Cherenkov counters of CLAS.
 - Line 252. Here you state 400 paddles for TOF. The number (as given above) is 342 paddles.
 - Line 260. The word "peculiar" is not appropriate. Perhaps you meant "particular"?
 - Line 266. I suggest some clarification. Use "These insensitive regions were located in the regions blocked by
   the torus coils, as well as ...".
 - Line 272. Here you use the word "efficiency". Really you mean "acceptance" and that is the better word choice here.
 - Line 274. The statement "Tracks with a polar angle $\theta \< 27^\deg$ in the forward direction were removed." is
   not correct (that would be the entire Forward Carriage!). Please review.
 - Line 278. Use "... in the six sectors, with the cut on $\theta$ being $\phi$-dependent.".
 - Line 284. Use "Timing information was used ...".

Page 4:
 - Fig. 2. I think you should remove the red vertical lines that extend beyond ~100 deg to avoid confusion on
   your fiducial volume.
 - Fig. 2 caption. Line 1. Use "... $\pi^-$ (middle), and protons ...".
 - Fig. 3 caption. Line 1. I do not care for your language of "real" events. I think it is more appropriate you
   use "data events" as compared to "Monte Carlo events". So, I would suggest to use "... for data (black) ...".
 - Fig. 3 caption. Line 3. Use "... to the number of data events.".
 - Line 291. Ref. [76] is not correct here. It should be [79].
 - Line 296. Use "... as for data events.".
 - Line 308. Ref. [75] is not correct here. It does not say anything about mechanical deformations of the tagger.
 - Line 310. Use "... of 0.52\%, which quantifies ...".
 - Line 322. Use "... $\lambda$, and $\phi$ ...".
 - Line 326. Use "... by the kinematic fit.". I think you should also eliminate giving Ref. [76] again here as it
   was just given on line 316.
 - Line 335. Use "... are rejected; 72\% of the ...".

Page 5:
 - Line 352. Use "... acceptance for data events ...".
 - Line 353. Use "... studies in Ref.~[76] that give ...".
 - Line 354. Use "... systematic uncertainty on the yield ...".
 - Fig. 4. None of the axes are labeled on these plots.
 - Line 358. Use "... of the uncertainty definition ...".
 - Eq.(4). End this equation with a period for proper punctuation.
 - Line 361. Use "When the uncertainties of the ...".
 - Line 377. Neither of the references given here ([56],[57]) mentions anything about an event generator. Are these
   references what you had intended here?
 - Line 378. Use "... were generated using a phase space ...".
 - Line 381. Use "... and $\pi^+$, with the missing mass computed under the assumption ...".
 - Line 382. When you discuss the "mean three-pion background, do you mean the average over all kinematic bins?
 - Line 394. Use "... and the requirement for the final state hadrons ...".
 - Line 396. Use "... to 5 variables, which we choose ...".
 - Line 397. Use "$M_{p\pi^-}$, and two angles ...".

Page 6:
 - Fig. 6: The notation S_rho is not defined and neither is the X(0+) exchange. Would is not be appropriate to label
   the figure in the middle left as s-channel and the figure in the lower left as t-channel.
 - Fig. 6 caption:
   - Line 1. Use "... system, the photon and proton ...".
   - Line 3. Use "$z$-axis".
   - Line 6. Use "... direction (center) and in the Gottfried-Jackson frame (bottom) the $\rho^0(770)$ is chosen
     as the $z$-axis".
 - Line 404. Use "$z$-axis".
 - Line 406. Use "... or in the direction of ...".
 - Line 409. Use "... the $z$-axis. Figure~6 shows ...".
 - Line 417. Use "coupled-channel".
 - Line 418. Use "database" in two places.
 - Line 419. Use "... all of the main ...".
 - Line 422. Use " {\it sample}, as well as the new ... [5], are included in ...".
 - Line 424. Use "database".
 - Line 425. Use "... imaginary parts of ...".
 - Table I caption:
   - Line 1. Use "coupled-channel".
   - Line 5. Include appropriate references relevant for the GWU and Karlsruhe-Helsinki groups.
 - Line 430. Use "event-by-event".
 - Line 432. It is not clear what you mean by "linear distributions" in relation to cross sections and asymmetries.
 - Line 437. Use "significantly".
 - Line 443. Use "... $D$-matrix that is based ...".
 - Line 448. Use "non-resonant".
 - Line 449. Use "... $K$-matrix poles. Further contributions ...".
 - Line 451. Use "... by the non-resonant terms.".
 - Line 453. Use "form factors".
 - Line 458. Use "These events were used to ...".
 - Line 463. Use "JLab-MSU".
 - Line 464. Use "... [58,59], which was successfully ...".

Page 7:
 - Fig. 7 caption. Line 1. Use "Comparison of the differential ... for the different missing particle topologies, ...".
 - Figs. 8, 9: Add a statement on the final fit curve. These figures would be better if the final fit curve was shown
   in color on the plots to separate it from the data points.
 - Figs. 8, 9 captions: Why the notation change from W to sqrt(s)? Use W as you have already introduced it.
 - Fig. 8 caption. Line 2. Use "... 2.1~GeV for the four ...".
 - Line 470. Use "... in Ref.~[4].".
 - Line 473. Use "... fits. Therefore we used ...".
 - Line 481. Use "... results far exceed the ...".
 - Line 482. Use "Hence, the ...".
 - Line 483. The sentence, "The acceptance of the event-based data sample is determined by the BnGa coupled-channel
   approach.", is misleading. The acceptance is not determined by the coupled-channel model but simulation model of
   the determine that was used. Please clarify your meaning.

Page 8:
 - Line 489. Use "... $\pi^+$, or $\pi^-$, reflect ...".
 - Line 492. Use "Thus, the mass and ...".
 - Line 501. Use "... of low cross sections.". Frequencies is not an appropriate word choice.
 - Line 505. Use "... $\pi^+$, and $\pi^-$ for ...".
 - Line 506. Use "... 0.04 in $\cos \theta$ for the ...".
 - Line 508. Use "... well accounted for by the ...".
 - Line 513. Use "... by crosses and the histogram represents the final fit.".
 - Line 522. The sentence states that a0 and a3 seem to prefer a lower mass. However, placing a ruler on the plot
   shows that the mass in a3 gives the largest overall value. What am I missing by your statement?
 - Line 525. Use "... widths of the $\Delta(1232)^{++}$ ...".
 - Line 548. Use "... fits to the individual ...".
 - Line 549. Use "... spread of the results ...".
 - Line 550. Use "... systematic uncertainty." (Drop "of the results" at the end.)
 - Line 553. Use "acceptance-corrected".
 - Line 555. Use "... in $W$ from 1600 to ...".
 - Line 569. Use "... resonance contributions".
 - Line 589. Here you mention "polarization data". This needs a reference to clarify what data you mean.

Page 9:
 - Fig. 10. The y-axis label of the right column of plots is missing.
 - Line 595. Use "... for the reaction ...".
 - Line 611. Use "... by a factor of 2, ...".
 - Line 615. Use "... the PWA uncertainty is due to ...".

Page 10:
 - Fig. 11 caption:
   - Line 1. Use "The black lines represent a fit ... only and the red lines a fit ...".
   - Line 2. Use "left-most".

Page 11:
 - Fig. 12:
   - The red CLAS data point have very strange looking/uniform error bars. These cannot be just statistical right?
   - The green circles and blue squares are not mentioned in the caption.
 - Fig. 12 caption:
   - Line 6. The red circle-dot symbol listed in the caption does not match the symbol used in the figure.
   - Line 7. Use "... ABBHHM Collaboration [28] that are consistent ...". By the way, you mention you compare here
     to the ABBHHM results, but there do not seem to be any ABBHHM data points on the figure.
   - Line 8. Use "The red, green, and blue ...".
   - Line 9. Use "... our result on the excitation ...".
   - Line 10. Use "... $\Delta(1232)^{++}$, and ... production, respectively, and the crosses ...". By the way, what
     are the "crosses" that you refer to here. There are no cross symbols on this figure.
   - Line 11. Use "... are the SAPHIR results.".
   - Line 14. Use " ... (see Ref. [32] for details).".
 - Line 651. Use "dominantly produced".
 - Line 667. Use "... as a function of the ...".
 - Line 668. Use "... and as a function of the cosine of the $\rho(770)$ cms scattering angle". Note that you are
   not consistent with your cms angle notation. In most places you just write "cos theta" but in the captions of
   Figs. 14, 16, and 18 you change notation and include "cms" in your notation. Be consistent throughout the paper
   to avoid confusion.
 - Line 670. Use "The $t$-dependence shows a fast exponential fall-off that is characteristic of diffractive
   scattering.".
 - Line 674. Use "... full cross section, leaving ...".
 - Line 676. Use "... differential cross section in the forward direction are ...". (Drop Nrho(770) as you stated this
   earlier in the sentence.)
 - Line 679. Use "... of the $N^*$ signals in this region.".
 - Line 681. Use "... or pion exchange that suggests resonance contributions.".
 - Table III caption. Line 2. Use "... and from the SAPHIR and ZEUS Collaborations.".
 - Table III. What are the units written in such a tiny font? Also for the next-to-last row, why is the "a" parameter
   not listed here for Ref. [53]?

Page 12:
 - Fig. 13. The right column title and x-axis label should use a lower case theta to be consistent with text notation.
   Also there are no "crosses" on the plots as mentioned in the caption.
 - Fig. 13 caption:
   - Line 1. Use "... as a function of $-t$ ...".
   - Line 3. Use "... define the uncertainty and range of the cross ...".
   - Line 4. Use "Pomeron".
 - Line 695. Use "The $\cos \theta_\rho$ distributions exhibit ...".
 - Fig. 14 caption:
   - Line 1. Use "The spin-density matrix ...".
   - Line 2. Use "... meson as a function of the ...".
   - Line 4. There are no bands on the figures, only two red lines.
   - Line 7. Include a reference to the SAPHIR data points.
 - Line 704. Use "The angular distribution of its decay into ...".
 - Eq.(5). End this equation with a period for proper punctuation and define the theta/phi angles in this expression.

Page 13:
 - Fig. 15 caption:
   - Line 2. Use "... decay as a function of the momentum ...".
   - Line 3. There are no bands on the figures, only two red lines.
   - Line 6. Include a reference to the SAPHIR data points.
 - Line 721. Use "... mesons as a function of $-t$ ...".
 - Line 722. Use "... frame; Fig. 15 is the same ...".
 - Line 733. Use "... frame that can be recognized ...".
 - Line 735. Use "... meson that scatters off a proton ...".
 - Fig. 16. The right column title and x-axis label should use a lower case theta to be consistent with text notation.
   Also there are no "crosses" on the plots as mentioned in the caption.
 - Fig. 16 caption:
   - Line 1. Use "... as a function of ...".
   - Line 2. Use "... define the uncertainty and range of the cross ...
   - Line 4. Use "... contribution and the dashed ...".
 - Line 740. Use "... as a function of $\cos \theta_ {\pi^-}$", where $\theta$ is the cms angle. The direct ...".
 - Line 743. Use "... role, which pion exchange ...".
 - Line 747. Use "Spin-density matrix elements:".
 - Line 748. Use "... 3/2$^+$, the elements ...".

Page 14:
 - Fig. 17. The x-axis label should use a lower case theta to be consistent with text notation.
 - Fig. 17 caption:
   - Line 2. Use "... decay as a function of $\cos \theta_{\pi^-} ...".
   - Line 3. There are no bands on the figures, only two red lines.
 - Eq.(5). End this equation with a period for proper punctuation and define the theta/phi angles in this expression.
 - Section IV.F. Figure 18 is not referenced in the text.
 - Line 763. Use "Only at higher energies does pion exchange play a significant ...".
 - Fig. 18. The right column title and x-axis label should use a lower case theta to be consistent with text notation.
   Also there are no "crosses" on the plots as mentioned in the caption.
 - Fig. 18 caption:
   - Line 1. Use "... as a function of $-t$ ...".
   - Line 2. Use "... define the uncertainty and range of the ...".
   - Line 4. Use "The dashed curves gives show the pion-exchange contribution and the dotted curves show the Born term.".
 - Line 766. Use "Spin-density matrix elements:".
 - Line 768. Use "... for $\gamma p \to \Delta(1232)^0 \pi^+$ in Fig. 19 also show significant ...". (Note you have the
   wrong figure referenced here in the text.)
 - Line 774. Use "... as a function of the squared momentum transfer $-t$ (see Fig. 13) show some ...".

Page 15:
 - Fig. 19: The x-axis label should use a lower case theta to be consistent with text notation.
 - Fig. 19 caption:
   - Line 2. Use "... decay as a function of $\cos \theta_{\pi^+}$ calculated ...
   - Line 3. There are no bands on the figures, only two red lines.
 - Line 786. Use "surprising".
 - Line 805. Use ".. that have existed so far ...".

Page 16:
 - Line 818. Use "coupled-channel".
 - Line 829. Use "coupled-channel".

Bibliography:
 - Put your references in the order cited in the paper.
 - [78] Use "R. Koch, and E. Pietarinen, ...".
 - [85] should be removed as it is not referenced in the text.