[Clascomment] OPT-IN:Measurements of ep -> e'pi+pi-p' Cross Sections with CLAS at 1.40 GeV < W < 2.0 GeV and 2.0 GeV2 < Q2 < 5.0 GeV2

[Daniel Carman]

Dear Evgeny et al:

I have read through your draft of the two-pion cross section paper and include 
my comments below. In general, I could follow the discussion and the main results, 
but there are a few places where the text could be clarified to improve the
discussion. In addition, there are a myriad of necessary edits to improve the 
grammar, syntax, and notation that I have tried to catch to lead to an improved 
version of the manuscript. If you have any questions, let me know. 

Regards,
Daniel
--------------------------------------------------------------------------------
General:
 - You are not consistent with how you write the main reaction of interest. There
   were at least four different notations used: ep --> ep'pi+pi-, ep --> e'p'pi+pi-,
   ep --> e'pi+pi-p', and ep --> e'pi+pi-p. Choose one and be consistent throughout.
 - You are not consistent with your usage of speed of light units throughout. I
   recommend you let c=1 and repair the inconsistencies on pp. 3 and pp. 14.
 - When referring to sections of the paper, use "Section" and not "section".
 - You are not consistent with your usage of "dimensional" and "d" and switch back
   and forth. Choose one and be consistent with your notation.
 - Whenever you quote numbers in the text for yields, they are written in a strange
   manner, e.g. "336 668" or "3 606 120". Please fix this.
 - The use of verb tense throughout this paper is not consistent. You should nominally
   write in the past tense. I have tried to catch this in my comments below.
 - You are not consistent in your usage of "electroproduction" or "electro-production".
   Be consistent throughout your manuscript.
 - Inconsistent usage of "3-momentum" vs. "three momentum" and "4-momentum" vs. "four
   momentum".

Main Physics Issue:
 - The main results of this paper, namely that the resonant contributions of the
   cross section seem to increase relative to the non-resonant contributions with
   increasing Q2, is based on comparison of predictions of the measured cross sections
   with the JM16 model. The trouble is that the JM16 model version shown in the paper
   does not include any projected model-dependent uncertainties. The comparisons seem
   to be all the more tenuous when there are potentially significant extrapolations made
   of the already measured Q2 evolution of the electrocouplings. These extrapolations are
   mentioned but not discussed. It seems that this paper could benefit from including
   this discussion as well as making an attempt to assign some sort of conservative
   uncertainties on the JM16 model "predictions".

Page 1:
 - Abstract. Line 2. Use consistent significant figures for listing the W range.
 - Line 21. The third paragraph is misplaced and contains a repetitive statement. It should
   go before the second paragraph (dropping the repetitive sentence) as follows:
   "Many nucleon states in the mass range above 1.6~GeV are known to couple strongly to
    $N\pi\pi$. Therefore, studies of exclusive $\pi+\pi-p$ electroproduction are ... "
 - Line 29. Use "... extracted from CLAS have ...".
 - Line 50. Use "(DA's)".
 - Line 65. Awkward structure. Use "... at distance scales of $Q^2 > 2$~GeV$^2$ where the
   quark core is the biggest contributor to the $N^*$ structure.".
 - Line 75. This sentence should not begin a new paragraph.
 - Line 79. Use "CLAS Collaboration".

Page 2:
 - Line 89. Use "... on these electrocouplings ...".
 - Line 94. I would move the placement of Ref. [1] to the end of the sentence. It is
   not appropriate in the middle.
 - Line 111. Use "... fractions for decays to $N\pi$.".
 - Line 112. Ref.[20] is definitely not "new". Use "Analysis of the combined
   $\pi^+\pi^-p$ photo- and electroproduction data [20] revealed ...".
 - Line 116. Use "... of its $\gamma_vpN^*$ electrocouplings, ...".
 - Line 118. The statement "This is the only candidate state for which information on
   ... become available." is not true. Electrocouplings have been extracted for a number
   of states. Please clarify your statement.
 - Line 130. Use "... in the range of ...".
 - Line 141. Use "... analyses of the exclusive ...".
 - Line 142. Use "hadroproduction".
 - Line 144. Why the notation change to $\sqrt{s}$? Use "W".
 - Line 145. Match significant figures. Use "... from 1.4~GeV to 2.0~GeV ...".
 - Line 146. Awkward structure. Use "... from 2.0~GeV$^2$ to 5.0~GeV$^2$ in terms of nine
   independent one-fold differential and fully integrated $\pi^+\pi^-p$ cross sections.".
 - Line 151. Use "kinematic".
 - Line 155. Use "... differential cross sections.".
 - Line 168. Use "The CLAS spectrometer consists of a series of detectors in each of its
   six azimuthal sectors, including ...". (Note Ref.[27] was already given and does not
   need to be repeated.)
 - Line 172. Use "... of particles, and a set of ...".
 - Line 173. Use "... to record the flight time of charged particles.".
 - Line 174. Use "... data acquisition triggered on a coincidence between signals in the 
   CC and EC, as explained below." No need to state the CC threshold as you do not give
   thresholds for any other subsystem.
 - Line 179. Use "Selection of Electrons".
 - Line 180. Use "The particle tracks were ...".
 - Line 182. Use "$z$-axis".
 - Line 191. Use "... electron produced an ...".
 - Line 193. Use "... shower were different ...".
 - Line 194. Use "... shower was not ...".
 - Line 195. Use "... so it was necessary ...".
 - Line 196. Use "... to remove these unwanted ..."

Page 3:
 - Fig. 1 caption. Line 2. Use "The vertical lines show ...". (They are not dotted.)
 - Line 199. Use "The edges of the fiducial regions were chosen based on studies ...".
 - Line 201. Use "... elastic $e-p$ scattering.".
 - Line 205. Use "Normally incident minimum ionizing pions typically lost ...".
 - Line 208. Use "... electrons underwent an ...".
 - Line 209. Use "... shower, which deposits ...".
 - Line 210. Use "... eliminated most of ...".
 - Line 212. Use "... electrons came from the ...".
 - Line 216. Use "... data formed a Gaussian ...".
 - Line 218. Use "... was applied to ...".
 - Line 219. Use "... tail were accounted for ...".
 - Line 220. Use "... cut was placed ...".
 - Line 222. Use "... hadrons was obtained ...".
 - Line 225. Use "... momentum of the particle track ...".
 - Line 237. Use "... positively charged particles ...".
 - Line 238. Use "... negatively charged particles ...".
 - Line 240. Use "... of the DC or bad SC paddle PMTs." No need to spell out DC or SC again.
 - Line 242. Use "... positively charge pions ...".
 - Line 243. Use "... in a plot of the measured track momentum $p$ vs. the polar angle $\theta$
   of the track.".
 - Line 245. Use "... regions were cut out of both the data ...".
 = Line 247. Use "In addition, cuts were ...".
 - Line 249. Use "... which eliminated ...".
 - Line 250. Use "... of the DC.".

Page 4:
 - Fig. 3 caption. Line 2. Use "... which separates the minimum ionizing pions ...".

Page 5:
 - Fig. 4 caption. Use "Velocity difference $\beta_{TOF}-\beta_{DC}$ for the sample of
   positively charged tracks vs. momentum for assumed masses of a pion (top) and a proton
   (bottom)."
 - Fig. 5 caption. Use "Histogram of the correlation between the momentum $p$ and the
   polar angle $\theta$ for tracks of positively charged pions in Sector~3 of CLAS. The
   inefficient regions of the detector, shown between the bands of solid lines, are removed
   from the analysis.".

Page 6:
 - Line 253. Use "... proton, and positively charged pion ...".
 - Line 259. Use "... of the $\pi^-$ using the ...".
 - Line 262. Use "... width compared very well ...".
 - Line 265. Use "... electron radiated a ...".
 - Line 266. Use "... it scattered from ...".
 - Line 267. Use "... peak was calculated ...".
 - Line 268. Use "... and were corrected for ...".
 - Line 269. Use "... selections were applied, ...".
 - Line 273. Use "... and the squared four-momentum transfer ...".
 - Line 280. Use "$x-z$ plane".
 - Line 281. Use "$z$-axis".
 - Line 282. Use "$y$-axis".
 - Line 286. Use "... the final state proton $p'$, ...".
 - Line 291. Use "... final state hadrons ...".
 - Line 297. Use "... of the final state ...".
 - Line 297. Use "... hadron pairs, $M_{\pi^+\pi^-}$ and $M_{\pi^+p}$, respectively, ...".
 - Line 301. Use "... 5 kinematic variables. Indeed, 3 final state particles ...".
 - Line 306. Use "... final state particles, ... remaining kinematic ...".
 - Line 307. Use "... variables that determine ...".
 - Line 314. Use "... determined by the ...".
 - Line 316. Use "... the final state ...".
 - Line 321. Use "... from the 3-momenta ... the final state particles ...".
 - Line 325. Use "... otherwise, were defined ...".
 - Line 327. Use "The $M_{\pi^+\pi^-}$ and $M_{\pi^+p}$ invariant ... related to the 
   four ..."
 - Line 329. Use "... final state particle ...".
 - Line 1 after 329. Use "... between the 3-momentum of the initial state photon 
   and the final state $\pi^-$ in the CM frame was calculated as:"
 - Add period after Eq.(3).
 - Line 330. Use "... angle was defined ...".
 - Add period after Eq.(9).
 - Line 2 after Eq.(9). Use "... B was more complicated. First we determined ...".
 - Line 6 after Eq.(9). Use "... and the $\pi^-$ 3-momentum $\vec{P}_{\pi^-}$.".

Page 7:
 - Fig. 6 caption. Use "Square of the $ep \to e'p'\pi^+\pi^-$ missing mass, showing a
   peak at the $\pi^-$ mass squared. The dashed histogram is from the Monte Carlo and the
   solid histogram is the data. The vertical lines ...".
 - Fig. 7 caption. Line 2. Use "... $W$ and squared 4-momentum transfer ...".
 - Line 1. Use "... toward the $\pi^+$ 3-momentum ...".
 - Line 3. Use "... 3-momenta of the $\pi^+$, $\pi^-$, and ...".
 - Line 6. Use "... between the two planes ...".
 - Add a period after Eq.(11).

Page 8:
 - Fig. 8.
   - Plane C is not identified on the figure, only in the caption.
   - Caption line 2. Use "... description of the ...".
 - Line 1 of text. Use "In the case of anti-collinear ...".
 - Add period after Eq.(12).
 - Line 333. Use "Cross Section Formulation".
 - For Eq.(13), make it a proper equation. Use for the l.h.s.,
   \frac{d^7\sigma}{dWdQ^2d^5\tau} =. Also add a comma after the equation.
 - Line 1 after Eq.(13). Use "... $\Delta N$ is the number of events ...".
 - 2 lines above Eq.(14). Shouldn't this say "... from the hadronic 5-dimensional
   phase space ...">
 - Include equation number on the unnumbered equation between Eq.(14) and Eq.(15).
   Also, put a comma after the equation.
 - Do not begin a new paragraph with "where $\Gamma_v$ is the ...".
 - Add a comma after Eq.(15).
 - Line 1 above Eq.(16). Use "... the virtual photon polarization parameter, ...".
 - Add a period at the of Eq.(16).
 - Line 337. Use "here $\omega = ... electron polar angle ...".
 - Line 338. Use "... frame, and $W$, $Q^2$, and ...".
 - Line 342. Use "... between the $\pi^+\pi^-$ ...".
 - Line 343. Use "... final state hadrons ...".
 - Line 344. Use "... 7-d cells were kinematically ...".
 - Line 345. Use "They were populated ...".
 - Line 348. Use "... event, which makes it virtually ...".
 - Line 355. Use "... sections. In the first step ...".
 - Line 358. Here and elsewhere, do not use "cross-section" or "cross-sections". Remove
   the hyphen.
 - Line 362. Use "... the 5-fold differential ...".

Page 9:
 - In Eq.(17) (once) and Eq.(18) twice. Use $\cos$.
 - Line 370. Use "The statistical uncertainties for the 1-fold ...".
 - Line 371. Use "... 14\% at the smallest ... to 20\% ...".
 - Line 374. Use "... with the uncertainties achieved in our previous CLAS data ...".
 - Line 1 after line 377. Use "The Monte Carlo event generator employed for the acceptance 
   studies was similar to that described in Ref.[20].".
 - Line 2 after line 377. Use "This event generator was capable of simulating the ...
   for the major meson ...".
 - Line 4 after line 377. Use "... electroproduction channels in the $N^*$ ...".
 - Line 6 after line 377. Use "... parameters ($W$, ...".
 - Line 8 after line 377. Use "... also generated radiative ...".
 - Line 9 after line 377. Use "... according to Ref.[28].".
 - Line 11 after line 377. Use "JLab-MSU".
 - Line 13 after line 377. Use "... events were fed ...".
 - Line 16 after line 377. Use "... bin was then given by ...".
 - Add a comma after Eq.(19).
 - Line 383. Use "... evaluations were reliable.".
 - Line 384. Use "... cuts, the detector efficiency tables for a given kinematic bin were
   determined in order to calculate the cross sections.".
 - Line 387. Use "... cells, there were a reasonable ...". Also, replace "reasonable" with
   a proper quantitative statement.
 - Line 391. Use "... these kinematic regions. It was necessary to account ...".
 - Line 402. Use "... kinematic variables ...".
 - Line 403. Use "... generator gave a fair ...".
 - Line 405. Use "... over the kinematic variables for all ...".
 - Line 407. Use "... between the measured ...".
 - Line 408. Use "... agreement was achieved over ...".
 - Line 412. Add a comma after the 5-fold cross section differential.
 - Line 414. Use "... (we weighted these ...".
 - Line 415. Use "... inside the 5-d ...".
 - Line 419. Use "... from the event ...".
 - Add a comma after Eq.(20).

Page 10:
 - Fig. 9 caption.
   - Line 1. Use "... between the measured ... and the simulated ...".
   - Line 3. End sentence with a period.
 - Line 424 should not begin a new paragraph. Use "and the 5-fold differential ...".
 - Line 427. Use "... between the 1-fold ...".
 - Line 428. Use "... without the generated ...".
 - Line 2 of Section G. Why say "2 pion events" and not "ep $\to e'p'\pi^+\pi^-$ events"?
 - Line 5 of Section G. Use "... [28] was used.".
 - Line 6 of Section G. Use "... we integrated the ...".
 - Line 8 of Section G. Use "... considerably reduced the ...".
 - Line 9 of Section G. Use "... hadron kinematic variables on the ...".
 - Line 12 of Section G. Use "The radiative correction factor $R$ in Eq.(13) was determined
   as:".
 - Line 436. Use "... are the numbers of ...".
 - Line 439. Missing units on Q^2.
 - Line 440. Use "... as a function of $W$ in Fig. 11.".
 - Line 442. Use "... from lower $W$ to ...".
 - Line 443. Use "... $W$, and because ...".
 - Line 445. Use "... bump in the factor ...".
 - Section G. Fig. 11 focuses on a single Q^2 bin. Is this bump feature in the R factor
   seen for all Q^2? If Fig. 11 is just a representative bin, then this should be made
   clear.

Page 11:
 - Fig. 10 caption.
   - Line 2. Use "The one-fold differential ...".
   - Line 3. Use "... blind CLAS areas are shown by the black ...".
 - Line 447. Use "systematic uncertainty".
 - Section H. This whole discussion about luminosity being one of the main sources of
   systematic uncertainty is improper. The systematic uncertainty on the luminosity is
   relatively small, depending only on the uncertainty in the beam current and the target
   density/length. What you are actually talking about here is not luminosity, but
   efficiency (which includes geometric acceptance). Please revisit your language here.
 - Line 452. Use "... set allowed us to ...".
 - Line 454. Use "... This allowed us to combine the normalization, ...".
 - Line 456. Use "identification uncertainties".
 - Line 459. Use "... parametrized cross section is given after radiative effects have
   been applied, and ...".
 - Line 461. Use "... data were not corrected ...".
 - Line 464. Use "... assign a 10\% global scale uncertainty due to ...".
 - Line 466. Use "We restricted ...".
 - Line 468. There is some problem with your notation involving quotation marks here.
 - Line 468. Use "... caused some loss of events.".
 - Line 471. Use "The uncertainty associated ...".
 - Line 473. Use "... mass cuts applied both ...".
 - Line 475. Add a comma after the mass range.
 - Line 479. Use "We used the ...".
 - Line 481. Use "... we calculated the relative ...".
 - Line 483. Use "... We expected to see ...".
 - Line 485. Use "... between the centroid of this distribution ...".
 - Line 485. The systematic should not just be determined from the centroid of the difference
   distribution but also through consideration of the width of this distribution.
 - MC question. Did the resolution of the missing mass distributions match the data? This is
   important to specifically mention that adjustments were made to make them match.

Page 12:
 - Fig. 11 caption. Use "The radiative correction factor ...". Also end the sentence with a
   period and mention in the caption what the magenta line represents.
 - Line 486. Use "... measure of the systematic uncertainty.".
 - Line 488. Use "... cuts at about ...".
 - Line 490. Use "... of the detector ...".
 - Line 491. Use "... recalculated the cross sections without ...".
 - Line 492. Use "... we reconstructed ...".
 - Line 495. Use "... 2\% in the cross sections.".
 - Line 499. Use "... we applied at $\pm$2$\sigma$ ...".
 - Line 501. Use "... cross sections with a $\pm$3$\sigma$ ...".
 - Line 503. Use "... we saw a systematic ...".
 - Line 506. Again, not luminosity but efficiency.
 - Line 508. Use "The summary of the systematic ...".
 - Line 512. Use "... to fill the blind ...".
 - Line 514. Use "... was estimated ...".
 - Section H. It is not clear what systematic sources are point-to-point and what sources
   are scale. It seems like they should be separated and only the point-to-point shown on
   your cross section plots with the scale uncertainty limited to the text and caption.
 - Table I. Use consistent significant figures for your sources.
 - Table I caption. End the caption with a period.
 - Line 522. Use "... at $W$=1.5~GeV ...".
 - Line 524. Use "... in the $W$- ...".
 - Line 527. Use "... on the $\pi^+\pi^-p$ ...".
 - Line 530. Use "... virtualities $Q^2=5$~GeV$^2$, ...".
 - Be sure to put your data in the CLAS physics database and include the proper citation.

Page 13:
 - Fig. 12 caption.
   - Include Ref.[32] to the Bosted parametrization. Explain the regions in theta where there
     are missing data. Also use "... Bosted parametrization [32] as a function of electron
     polar angle $\theta$ for each of the six sectors of CLAS.".
 - Fig. 13 caption.
   - Use "Fully integrated cross sections for .. off protons at photon virtualities $Q^2$= ...".
   - End the sentence with a period.
   - What are the error bars shown on these data points?

Page 14:
 - Line 539. Use "... and the projected ...".
 - Line 543. Use "... in Ref.[16] and using interpolation of the resonance ...".
 - Line 554. Use "... on the resonance ...".
 - Line 555. Use "... found in Table II.".
 - Line 556. Use "... values, together with the appropriate references, are ...".
 - The discussion in lines 558 to 608 is not crisp. It is too repetitive. Please revisit these
   paragraphs. It seems that you keep talking about the extrapolations and interpolations 
   multiple times when it needs to be stated clearly once.
 - Line 559. Use "... resonant contributions ...".
 - Line 563. Use "... found in Ref.[34].".
 - Line 564. Use "... in the mass range ...".
 - Line 565. Use "... on the $\gamma_vpN^*$ ...".
 - Line 567. Use "... resonances were estimated ...".
 - Line 571. Use "... to the $\pi^+\pi^-p$ ...".
 - Line 580. Spurious "the".
 - Line 585. Use "... resonances were taken ...".
 - Line 590. Use "... allowed us to ...".
 - Line 597. Use "... the $N(1440)1/2^+$ and ...".
 - Line 598. Use "... resonances, as well as ...".
 - Line 599. Use "... state, are available in ...".
 - Line 601. Use "... electrocouplings were used ...".
 - Line 608. There is a spurious tilde over the GeV units.
 - Line 614. Use "... of the projected ...".
 - Line 616. There are three W intervals in Table IV not four.
 - Line 618. Use "... GeV, electrocouplings ...".
 - Line 633. Use "In Figs. 16, 17, and 18 ...".
 - Line 646. Type on "sizable".
 - Line 652. Use "... for the final state $\pi^-$ ...".

Page 15:
 - Table III caption. 
   - Line 1. Use "... to the $\pi^+\pi^-p$ ...".
   - Line 2. Use "... section off protons, ...".
 - Fig. 14 caption.
   - Line 1. Use "The resonant contributions from the JM16 model [4,16,18] computed as ...".
   - Line 1. Use "... CLAS results on the ...".
   - Line 3. Use "... mass of the ...". Also end the sentence with a period.
 - Table IV.
   - Use consistent significant figures for the resonant contributions and their uncertainties.
   - Caption line 1. Use "... over the measured ...".

Page 16:
 - Fig. 15 caption.
   - Line 1. Use "The resonant contributions from the JM16 model [4,16,18] computed as ...".
   - Line 1. Use "... results on the ...".
   - Line 3. Use "... mass of the ...". Also end the sentence with a period.
 - Fig. 16 caption.
   - Line 1. Use "The resonant contributions from the JM16 model [4,16,18] to the nine ...".
 - Line 655. Use "$t$-channel".
 - Line 661. Use "... ratio of the projected ...".
 - Line 660. This sentence makes absolutely no sense to me. The ratio increases with Q2 not
   decreases. Please revisit what you are saying here.
 - Line 665. Use "... range with resonant ...".
 - Line 667. Use "... from the non-resonant ...".
 - Line 674. Use "electroproduction".

Page 17:
 - Fig. 17 caption.
   - Line 1. Use "The resonant contributions from the JM16 model [4,16,18] to the nine ...".
 - Fig. 18 caption.
   - Line 1. Use "The resonant contributions from the JM16 model [4,16,18] to the nine ...".
 - As I mentioned above, my biggest issue with this paper is that there is no uncertainty
   given associated with the JM16 model. Without some conservative estimate as to the
   uncertainties, especially given the finite Q2 extrapolations being made to make your
   predictions, it is not clear how seriously to take any comparisons of the JM16 predictions
   against the data. All of your major conclusions hang on this comparison. In other words,
   your conclusions may turn out to be totally unreasonable if there are sizable uncertainties
   on the model predictions.

Page 18:
 - Line 685. Use "cannot".
 - Line 686. Use "... of the resonance ...".
 - Line 687. Spurious period at the end of the sentence.
 - Line 698. Use "... corresponds to ...".
 - Line 699. Use "... cloud, which becomes ...".
 - Line 708. Use "CONCLUSIONS".
 - Line 711. Use consistent number of significant figures in your Q2 range.
 - Line 713. Use "$N\pi$".
 - Line 714. Use "... data cover the ...".
 - Line 737. Use "... contributions from ... cross section at ...
 - Line 738. Use "... It suggests good ...".
 - Line 761. This sentence should not begin a new paragraph. It is a direct continuation of
   the previous thread.
 - Line 763. Use "... in the Dyson-Schwinger equation (DSE) approach ...".
 - Line 765. Use "... model predictions ...".
 - Line 769. Use "We express our gratitude ...".
 - Line 774. Use "... and Ohio ...".
 - Line 775. Use "... (OU). Jefferson Science Associates ...".


Page 19:
 - Ref.[3]. Use "... Mokeev, arXiv:1509:08523 [nucl-ex].".
 - Ref.[13]. Do not include arXiv number for published proceedings.
 - Ref.[18]. Format change. Use "V.I. Mokeev".
 - Ref.[27]. Use "B.A. Mecking ...".