[Clascomment] OPT-IN:Longitudinal target-spin asymmetries for deeply virtual Compton scattering

[Silvia Niccolai]

Hello Michel,

On Wed, 15 Oct 2014, Michel Garçon wrote:

> Hi Silvia et al.,
>
> Thanks for your detailed answers to my first comments.
>
> - As I told you before, there are some approximations in BMK (Dieter acknowledged
> this some years ago). Unfortunately, their new and more complete paper 
> BMJ is not so clear. And no, I am not sure that DVCS2 appear in sin(phi) 
> at leading order. Some terms are not clearly defined. So I take this 
> back for now. This being said, we know more now than in the early 
> papers. Especially if you do not mention "leading twist" in those 
> sentences, I would keep my comment: instead of "which are a signature of 
> the interference...", something like "which are expected to
> arise mostly from the interference..". This is not asking you to soften 
> a statement compared to what we said years ago, but to acknowledge 
> (implicitly) that we know more details now on the formalism. Again, if 
> you do not mention leading twist, DVCS2 contributes to the asymmetry 
> (sin2phi at least for sure).

The abstract has now been modified, the word "signature" has gone, 
the expression "leading-twist" has appeared. I'll send out soon the new 
version of the paper which will hopefully satisfy your concerns.

> - On the definition of CFFs, I wait to see the next version of the paper,
> but I would insist it would make more sense for our collaboration to 
> have a consistent convention in all its DVCS papers. With all due 
> respect to the authors of ref [8],of whom 2 are distinguished CLAS 
> collaborators, the definition of CFFs as 4 imaginary quantities, is a 
> more widely accepted one (among others eq. 17 in arXiv:1101.2482 - with 
> same author that introduced the other convention previously - , eq. 4 in 
> 9905372, eq. 9 in your ref. 6, etc...) and one that makes more sense.

Personally, I don't see the need to adopt the same exact definition of 
the CFFs for two CLAS papers published almost ten years apart. In 
FX's paper you cited only the H CFF, while in our case I needed a more 
"compact" definition (and Michel's serves this purpose) to embed both the 
unpolarized and polarized GPDs. The current version of the paper specifies 
that we adopt one of the definitions, with its ref. I think it is enough. 
There is no political reason behind this choice, just a practical one, 
and it doesn't bring any heavy consequence on the content of the paper.

> - Importance of H_tilde: you have not shown that H_tilde dominates the
> TSA. I continue to think (and it was for example in ref.13) that the H 
> and H_tilde contributions are of the same order of magnitude (I will not 
> argue here on the exact value of the ratio, I was just giving back of 
> the envelope estimates). The only way you can see that is by turning 
> off and on each contribution in VGG for example. So I agree of course 
> that TSA is sensitive to H_tilde, but any statement that (implicitly) 
> leads to believe it is uniquely sensitive to H_tilde is misleading. It 
> is the case on line 265 where it is said that BSA and TSA are mostly 
> sensitive to H and H_tilde respectively. The truth is, BSA is mostly 
> sensitive to H, and TSA gets its main contributions from both H and 
> H_tilde, thus adding a sensitivity to H_tilde. Thank you for the CFF 
> plots, but they do not disprove the above. You have to look at the 
> relative contributions to substantiate any statement (+ the relative 
> errors on ImH and ImH_tilde from Michel's fit are not so different).
>

I ran VGG at your request, with and without Htilde, you can find the 
result in the following plot that Erin made:
http://www.jlab.org/Hall-B/secure/eg1-dvcs/erin/publication/ 
tVGG_alpha_no_htilde.eps

VGG with Htilde is the red curve, without it's the green curve. There is 
about a factor of two among the two, but yes, the asymmetry doesn't drop 
to zero, which means that there is still an important contribution from H 
in this observable. I have modified the text of the paper, and whenever 
the Htilde dominance of the TSA was mentioned now there is also H nearby. 
I don't think this modifies the conclusions in any way, as the difference 
in slope BSA-TSA must come from Htilde.

> Looking forward to the next version.

I'll send it to the Ad Hoc and then it will be posted for the final author 
check upon their approval.

Thank you for your contribution and interest for this paper.

Best regards,
Silvia