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

[Stepan Stepanyan]

Hello Hyon-Suk,

Thanks for the reply. Just two comments on my last two points:
- Since I do not know what that mean "kinematic-independent" I can argue 
that you could have use
radiative elastic events for your renormalization factor. In 
ep->epX(\gamma), where gamma is radiated
from incoming electron, your electron and proton kinematics are very 
similar to your DVCS events.
By the way, I do not believe that effects that simulation cannot 
reproduce can be "kinematic-independent".

- OK, your x-bins are small and you end up having only 2 Q2 bins, but as 
I can see in the x-bin close to
the value of Hall-A x-value you can have one point with in average 
higher Q2. So, why not to put this
one point, extend Hall-A Q2 dependance and see if it stays flat.

Stepan

On 3/31/15 12:17 PM, Hyon-Suk JO wrote:
> Hi Stepan,
>
> Thanks for your comments.
>
> 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 
>
>
>> - page 2, line 105, the CLAS calorimeter polar angular range for 
>> photons is 8 to 45, it became 15-45 due to IC. The sentence should be 
>> corrected, to clarify that
>
> On lines 104-108, we replaced "The polar-angle acceptance
> of CLAS for photons (~15-45 deg) was extended down to 5 deg
> with the addition of a specially designed electromagnetic
> calorimeter ("inner calorimeter", IC [13]), with full
> azimuthal coverage." with "A specially designed
> electromagnetic calorimeter ("inner calorimeter", IC [13])
> was added to the CLAS detector and allowed the detection
> of photons for polar angles from about 5 deg to 16 deg,
> with full azimuthal coverage". We will simply omit the
> polar-angle coverage of the EC as we discarded the photons
> detected in the EC from our analysis.
>
>> - page 2, line 144, sentence "In order ... we chose to minimize the 
>> size of our bins, ...". Can you give the value of minimized bin size 
>> the resolution of the detector for that variable
>
> As the unpolarized cross section varies strongly as a
> function of xB, we defined most of our bins in xB with a
> size of 0.03 or 0.04. Our average resolution in xB is 0.005
> while our average resolution in Q2 is 0.035 GeV^2.
>
>> - page 3, line 184, "... of 12.3%, meant to compensate for various 
>> effects that 185 are not well reproduced by the simulations". This 
>> means it either comes from efficiency calculation or radiative 
>> effects. Both effects will be very different for elastic events. is 
>> not it. How elastic with very different kinematics can be used for 
>> this arbitrary correction
>
> 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.".
>
>> - one of interesting results from Hall-A measurement was Q2 
>> independence of the Im-part of the Compton FF, that was attributed to 
>> twist-2 dominance. It seems we have much more data to say something 
>> with respect to this important question. Why we are not giving Q2 
>> dependence? Is it because there is not enough statistics in any of 
>> x-bins to have few Q2 points?
>
> Because of the small size of our bins in xB, we had to
> adopt larger bins in theta_e (for enough statistics), only
> leading to two Q2 values for each bin in xB. This is indeed
> the reason why we could not extract a Q2 dependence similar
> to what was done in the Hall A paper.
>
> Cheers,
> Hyon-Suk