[Clascomment] OPT-IN:Measurements of $gamma_{v} p ightarrow p' pi^{+} pi^{-}$ cross section with the CLAS detector for $0.4 GeV^{2} < Q^{2} < 1.0 GeV^2$ and $1.3 GeV < W <1.825 GeV$
Victor Mokeev
mokeev at jlab.org
Mon Feb 19 13:29:56 EST 2018
Dear Colleagues,
In the recent paper on pi^+pi^-p electroproduction off protons by G.Fedotov, Iu. Skorodumina, et al., my previous questions on reliability of the statistical error bars for the experimental data were fully addressed. I think it is important for us to provide appealing presentation of these new results in order to maximize their impact on hadron and strong QCD physics.
I have both ``Major physics comments" and complementary ``Editorial suggestions". I have strong feeling that the ``Major physics comments" should be implemented perhaps with edits. In a case of disagreement on implementation of the major comments, I appreciate to know the reasons.
Major physics comments
1. p.1 line 24
Ref [1] should be extended as:
Ad1. V.D. Burkert, Eur. Phys. J. Web Conf. 134, 01001 (2017).
Ad2. V.D. Burkert and C.D. Roberts, arXiv1710.02549[nucl-ex].
Ad3. Iu. A. Skorodumina et al, Moscow Univ, Phys. Bull,70, 015203 (2015).
Ad4. I.G. Aznauryan and V.D. Burkert, Prog. Part. Nuvcl. Phys. 67, 1 (2012).
These references in Introduction, in my view, should introduce the place of our paper in the field of the N* physics.
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2.p. 1 line 63
high sensitivity-----> essential sensitivity.
Please note that for N(1440)1/2+, N(1520)3/2-, N(1535)1/2-, N(1675)5/2-, and N(1680) 5/2+ resonance which are heavier than \Delta, the Npi exclusive channels are the driving source of the information on their electrocouplings.
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2. p.15 line 925
after ``...are reported" add
Full data set is available in the CLAS Physics Data Base [2] (reference for the current paper version)
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3. p.17 left and right
Remove the text between lines 988-998 ``Beside that.... from Ref [7]" (see justification in my previous e-mail, which is attached below)
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4. p. 17 line 985
The electrocouplings of these nine states---->
The electrocouplings of the relevant nucleon resonances in the investigated Q^2 range were taken from fit of the available results [Ad5] on Q^2-dependencies of resonance electrocouplings extracted from the CLAS pi^+n, pi^0p, \eta p and \pi^+\pi^-p exclusive electroproduction off proton data [7,9,26-35]
Ad 5 https://userweb.jlab.org/~mokeev/resonance_electrocouplings/
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5. p. 17 before ``For all resonance states..." add
Electrocouplings of the excited nucleon states in the mass range up to 1.6 GeV are currently available at photon virtualities 0<Q^2<5.0 GeV^2. In computation of the resonant contributions they were estimated by interpolating the experimental results [Ad5] onto the Q^2-grid of our pi^+pi^-p data. The results on longitudinal electrocouplings S_{1/2} for most nucleon resonances with masses above 1.6 GeV are limited by the photon virtualities Q^2>0.5 GeV^2. For these high mass resonances, A1/2 electrocouplings were determined by interpolating the available experimental results including those at the photon point. Instead, S1/2 electrocouplings were interpolated at photon virtualities Q^2>0.5 GeV^2, while within narrow Q^2-interval 0.4 <Q^2< 0.5 GeV^2 we extrapolated their values assuming that they are equal to the interpolated values at Q^2=0.6 GeV^2 for each resonance.
All approximations used in the evaluation of the resonant contribution should be written down.
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6. p.17 lines 1011-1012
...consistent with previous studies [5,6]------------->
consistent previous studies [5]
Studies [6] cover Q^2>2.0 GeV^2. Resonant contributions at Q^2>2.0 GeV^2 and at Q^2<1.0 GeV^2 are just incompatible. There is no way to confront them.
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Editorial suggestions
1. p2. line 77
the JM model [7]----> the JM model [7,9,12,16]
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2. p.3 line 160 Abbreviations DC, CC, TOF, and so on, should be defined before not after their first use un the text.
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3. p. 3 line 181 \pi^- losses ----> \pi^- ionization losses
Note that pi^- can suffer also nuclear interactions producing high energy tail for the deposited energy
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4. p3 line 194 and Fig. 3 I was unable to see the vertical line in Fig. 3
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5. p.5 line 294
the momentum-----> the measured momentum
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6. p. 5 lines 315-319 ```Here, due to....can be neglected"
This part is not fully clear, in particular for outsider-readers.
Which total momentum we are speaking about, in which frame? In the CM, the total momentum of the final hadrons or the initial photon and proton is equal to zero. The fraction of zero makes no sense. Total momentum of the final hadrons should be equal of the total momentum of the initial photon and proton for any reaction. So, if we are speaking on the fraction of the energy-momentum transfer through the virtual photon which is carried out by each final hadron, it is in fact smaller in Npipi in comparison with Npi, while the absolute values of the final hadron momenta are fully determined by the W and by the five kin. variables for the final state kinematics. I propose to re-phrase this paragraph making it more clear.
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7. p.6 line 372
I propose to define more quantitatively ``...a relatively flat particle density"
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8. In Fig. 10 there is depletion at theta ~ 10-20 deg. How we treat this depletion? If it is outside the fiducial cut, may be the fiducial cut should be shown in Fig. 10?
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9. Fig. 11 ...blocks as function of DAQ time----> ...blocks versus DAQ time
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10 Fig. 12. It is unclear to which integral the distributions are normalized
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11 p.9 lines 567-568 Replace the text ``The binning size...Q^2 bins" as:
The binning over the final hadron variables is listed in Table I. It was chosen as compromise between the minimal bin size over kinematics variables and affordable statistical accuracy.
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12. Despite all my efforts, I was unable to understand the text between lines 597-609 in p.10. If possible, please write it in more clear form.
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13. p.11 lines 683-691.
As it is written, the paragraph is contradictory. If GENEV is using phase space, it does not use the JM05 model. I guess, pi^+pi^-p channel was simulated within JM05, while for three-pion background the phase space was used. Please, rephrase the paragraph making it self-consistent.
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14 p. 12
I strongly recommend to replace
empty cells----> blinded cells
or any other English word differentiating the cells of zero acceptance from empty not populated by the measured events cells.
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15 Better to present the summary Table of systematic uncertainties similar as done in Ref [6].
In addition:
in p. 17 lines 988-998 we have the statement which should be removed from the paper text.
First, we have no P13(1700) resonance at all, we do have P11(1710) or N(1700)1/2+ in the PDG notation. The contribution from \Delta(1600)3/2+, N(1675)5/2-, and N(1710)1/2+ to the pi^+pi^-p electroproduction off protons at Q^2<1.0 GeV^2 is inside the data uncertainties. We never have information on these state electrocouplings at Q^2<1.0 GeV^2 from the CLAS data. References [7,33] in the paper text (lines 988-998) are just irrelevant. The paper [7] reports electrocouplings of N((1440)1/2+, N(1520)3/2-, and \Delta(1620)1/2- states, but DO NOT report anything on \Delta(1600)3/2+ or P33(1600) state. The paper [33] does report electrocouplings of N(1675)5/2-, N(1710)1/2+ states BUT at Q^2>1.7 GeV^2, while the Q^2-coverage of the pi^+pi^-p electroproduction data in Fedotov/Skorodumina paper is limited by Q^2<1.0 GeV^2. Moreover, according to Fig.19,22 in Ref [33], electrocouplings A1/2 of N(1675)5/2-, and N(1710)1/2+ resonances demonstrate pronounced Q^2-dependence in contrast with the st
atement in lines 988-998 in the Fedotov/Skorodumina paper on their Q^2-independence. At W<1.8 GeV covered by Npipi data, the contributions from the tails of the \Delta(1905)5/2+ and \Delta(1950)7/2+ are inside the data uncertainties.
In my view, the best way to proceed with this problem is: just to remove the text between the lines 988-998
``Beside that the states.........taken from Ref [7]"
The paper contains the new pi^+pi^-p of the best quality ever published for Npipi electroproduction. For this reason this paper should be published, but without confusing statement on data interpretation, which does not affect the paper core, that is the presentation of the new data set.
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