[Clascomment] OPT-IN: Cross sections for the exclusive photon electroproduction on the proton and Generalized Parton Distributions

[Hyon-Suk JO]

Hi Daniel,

Thanks for your comments.
All your corrections and suggestions have been implemented.

The revised version of the paper can be found here:
https://www.jlab.org/Hall-B/shifts/admin/paper_reviews/2015/dvcs_cross_sections_prl_draft_v4-5413120-2015-03-31-v4.pdf

Here is our answer on the global renormalization factor
(already included in the answers to Stepan's comments):
===
We agree with you that this correction is probably the
weakest point of this analysis. However, it was agreed
within the e1-dvcs group to adopt it consistently in all
the cross section publications [see for instance
I. Bedlinskiy et al., Phys. Rev. C 90, 025205 (2014)]. This
factor accounts for problems in the efficiency evaluation,
and not in radiative corrections (as the elastic factor
was computed from the ratio of the measured _radiated_
elastic cross section compared to Cole Smith's calculation
of its theoretical value). So, efficiency problems for the
double-arm elastic channel should be common to the "ep" part
of the DVCS channel. It is true that the kinematics of the
two reactions are different, and that there is not much
overlap for either of the two particles. Nonetheless, it
was the only well-known reaction that we could use to cross
check and correct our normalization. This factor includes
the efficiency of the SC counters which was estimated to
be around around 95%, as well as other efficiency factors
which are not accounted for in the Monte Carlo, such as
trigger efficiency. We quote a big systematic on this
factor, among the biggest ones that we have, to account
for the limitations of this correction. On lines 183-188,
we replaced "In addition, we applied a global
renormalization factor of 12.3%, meant to compensate for
various effects that are not well reproduced by the
simulations. This factor was determined from the analysis
of the elastic scattering ep->e'p', comparing the
experimental cross section to the well-known theoretical
one." with "In addition, we applied a global
renormalization factor of 12.3%, determined from the
analysis of the elastic scattering ep->e'p', by comparing
the experimental cross section to the well-known
theoretical one. This factor compensates for various
kinematic-independent inefficiencies, not well reproduced
by the simulations.".
===
We hope that the change in the text "for various
kinematic-independent inefficiencies, not well
reproduced by the simulations" instead of "for various
effects that are not well reproduced by the simulations."
is a good enough clarification.

As for your comment on the future constraints provided by
these new data, here is our answer (already included in the
answers to Sebastian's comments):
===
Regarding the comparison of the models, the standard GPD
models indeed describe the data well in the six bins that
we show but the agreement is not equally good in all our
110 (Q2, xB, t) bins. We are giving average chi2 values to
give a general indication of how the models compare to the
data but in fact, the chi2 varies for each model as a
function of the bins/kinematics. Thus, one should not
conclude that, among the standard GPD models (VGG, KMS,
KM10a), one model is doing a better job than another as the
situation can differ in each bin. One should also note that
none of the four models discussed have been tuned to these
new data and that each of them can certainly adjust some
parameters and achieve a better agreement. The main point
of the comparison of the models with the data is the fact
that the data appear to be incompatible with the KM10 model
that includes the very strong H_tilde contribution. Because
of that, we might have oversimplified one of the
conclusions, saying that the other three models "describe
the data well". In order to take into account your remark,
we therefore add on lines 231-232 the sentence: "Note that
none of these four models has been tuned to our data.".
We also add the reference [33] as a footnote: "The chi2
values, integrated over all the bins, give a general
indication but the level of agreement or disagreement
between each GPD model's predictions and the data varies
as a function of the kinematics.".
===

Cheers,
Hyon-Suk

> ************************************************************************************
> Page 1.
> Line 9. Use "... still remain a mystery ...".
> Line 19. Awkward structure. How about "... launched worldwide at Jefferson Lab (JLab),
>   COMPASS and HERA, facilities using multi-GeV electromagnetic probes, to study ...".
> Line 50. Use "... of such a process." or "... of such processes.".
>
> Page 2.
> Line 87. You use different significant figures for the Q2 range limits and for the xB
>   range limits.
> Line 104. Need a space between quantity and units.
> Lines 124,125,126,128,130. You have introduced the prime notation for the final state
>   electron and proton. Therefore you should be consistent with this notation on your
>   missing mass label and your reaction labels in these lines.
> Fig. 2 caption. Line 6. Same comment as previous.
>
> Page 3.
> Line 163. Use "On average, ...".
> Line 168. Use "200 million".
> Line 184. "... meant to compensate for various effects that are not well reproduced
>   by the simulations." This factor is large enough (and worrisome enough) that I
>   would like to see you include a bit more information, such as the types of effects
>   that you are talking about.
> Line 186. Using elastic scattering (which is measured over a relatively narrow range
>   of kinematics) to correct for reactions (DVCS/BH) measured over a relatively large
>   range of kinematics, seems worrisome in regards to assigning inappropriate systematic
>   uncertainties.
> Lines 198,200,203. Use "on average".
>
> Page 4.
> Line 273. Awkward structure. How about "... simultaneously the $\phi$ distributions of
>   our unpolarized and beam-polarized cross sections at a given ...".
>
> Page 5.
> Line 323. "This considerable set of new data will provide stringent constraints on
>   GPD models". But you have shown in Fig. 4 that pretty much every model well describes
>   the measured data. That does not give me a sense that these data provide very much
>   of a constraint. Plus the discrepancy with the Hall A data kind of muddies the waters
>   on just how much of a constraint either of these datasets actually can provide.
>
> References.
> [15]. The paper is now published.
> [18]. Is not a CLAS Collaboration paper.