[Clascomment] OPT-IN: Exclusive pi-0 electroproduction at W > 2 GeV with CLAS

[Michel Garçon]

M. Garçon, April 2nd, 2014
Comments on « Exclusive pi0…. » by I. Bedlinskiy et al.
(collaboration wide review)

Overall very good paper.
I am missing :
-	A statement that this paper expands on the work of Ref.[13], giving a comprehensive description of the data analysis and giving complete results on this reaction channel from the same experiment.
Optionally:
-	In the discussion of the slope b, it strikes me that the Q2 dependence might be the same as for vector meson production. The concept of formation length should not apply here, except in the Regge theory if the rescattering diagrams are dominant. Could you check if b has a similar dependence as e.g. Eq.(4) and Fig.13 of EPJA 24 (2005) 445, or similar plots for the rho production (Fig. 15 of EPJA 39, 5)?  And if appropriate add a discussion about this ?
In several instances, there are repetitions between the text and a figure caption: a given statement/information should appear only once.
Editing (by page/line  numbers):
-	1/14-17: not an encouraging statement! Could be read as “progress is hopeless”. Makes me think of the usual critics about nuclear structure from the part of hadron or particle physicists. What about: “How partons move and how they are distributed in space is still an open question on which new theoretical and experimental developments are starting to shed a new light”.
-	1/31: “partons transverse positions and longitudinal momenta”.
-	1/66: there is really only one experiment here. Suggested change: “…..large virtuality Q2, at least one DVCS observable appears to exhibit at Q2 as low as 1.5 GeV2 a scaling behavior characteristic of the leading twist description of the handbag mechanism[6].”
-	1/80: “At a given Q2 scale, the GPDs depend on….”
-	2/34-35: remove “, especially….HERMES” (repetition from the beginning of the sentence).
-	3/32: replace detector by spectrometer.
-	3/52-53: “… 424 PbWO4 tapered crystals pointing 20 cm upstream the target position.”
-	3/81: Remove “1 x”.
-	3/84-85: I would remove this info on the integrated luminosity: only the usable or used one is significant here.
-	3/86-87: instead of “Runs with…excluded”, I would put “After applying strict run-to-run stability criteria, the integrated luminosity corresponding to the data presented here was 1.9 fb-1”.
-	Fig.6: put different colors on the different lines and explain in the caption what they correspond to. Would be nice if you could restrict the grid to the actually used bins.
-	5/5-7: you can be proportional and small, so the sentence needs to be reworded: “…deposits an energy in the calorimeter proportional to its momentum, while pions typically lose a small and fixed amount of energy corresponding to a minimum ionization process.”
-	5/13-14: repetition from the Fig. 8 caption. Choose one or the other.
-	5/16-21: left….right -> upper….lower 
-	5/36: repetition from Fig.10 caption. Are you sure it is an insulating foil, and not the vacuum chamber window?
-	Fig. 7 caption: the last sentence is irrelevant (nobody can use this information). I would add to the first sentence “in arbitrary units” and to the second one “normalized to each other”.
-	Fig.8: solid red lines need to be thicker.
-	10/30: why “overall” ?
-	Table IV: make a distinction between bin-to-bin systematic uncertainties and (what I would call overall) normalization systematic uncertainties.
-	12/after 37: add “For the remainder of this paper, sigma always refers to the reduced, twice-differential, cross section of Eq. (4).”
-	Fig.15: the xB values are given both in the graphs and in the caption. One or the other.
-	Same for Fig. 16 and Q2 values.
-	15/39: remove “below”
-	Fig.19: is the statistics box needed? Useful? Also the last sentence of the caption is also in the text.
-	17/14+: “In Ref. [20], a vector-meson rescattering amplitude was included. It takes the form….”
-	Figs 20 and 24: individual data points need to be bigger.
-	20/1: it is not “the relative contribution of ET to HT” but “the contribution of ET relative to the one of HT”
-	26/39-40 + 22/1-4: the sentence is a bit heavy with 2 “which”.
-	26/14-16: not the place in the conclusion to make this remark, which was already made before.

Other (not related to the paper itself):
    On the list of reviews for the CLAS collaboration, this is listed from e1f data set. It should be e1-dvcs.