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

[Silvia Niccolai]

Hi Stepan,

On Tue, 7 Oct 2014, Stepan Stepanyan wrote:
> The fact that you cannot expand descriptions in the abstract, it does not 
> mean you can get that much short cut.
> In some reactions actually, higher twist effects may be larger than leading 
> twist. How about this:
> "For the first time target-spin asymmetry for the $e~p \to e^\prime p^\prime 
> \gamma$ reaction, which in leading twist is a  signature of the interference 
> of the Deeply Virtual Compton Scattering and the Bethe-Heitler processes, 
> ..."

While I agree on including clarifying sentences about the higher-twist in 
the text (I am working on it...), I am still a bit reluctant in including 
the concept ot "twist" in this position in the abstract. Our Ad Hoc 
committee asked us to give a handwaving definition of "leading twist" in 
the text, because it seemed a too obscure concept for the general PRL 
reader. Moreover, I looked at the abstract of the previous CLAS DVCS 
papers (yours, FX's and Shifeng's), all done at basically our same 
kinematics, and no one of them said in the abstract that the fact that 
observing an asymmetry is a signature of the DVCS/BH interference is true 
***only at leading twist***. 
I propose you the following compromise solution for the abstract, that 
hopefully could also partially address the following comments on axial 
charge and higher-twist stuff: leave the part referring to the 
interference as it is, and changing the third sentence as follows:
"In the framework of Generalized Parton Distributions (GPDs), at leading 
twist, the $t$ dependence of these asymmetries provides insight on the 
spatial distribution of the axial charge of the proton, which appears to 
be focused in its center."

> The way it is written it is not clear that you are talking about offline 
> analysis. By the way, did you use cut on number of
> photo-electrons in CC? or only energy cut in EC, is it only total energy cut 
> to reject negative pion?

For the wording of this part, we followed the suggestions of our Ad Hoc 
committee. I'll try and see if it can be made clearer that the EC cuts are 
done offline. 
No, we don't cut on nphe(CC). We cut on EC_inner, cut on the time 
difference between SC and CC, do EC fiducial cuts. The nphe spectrum looks 
nice and clean after these cuts and the exclusivity ones, no need to cut 
on nphe>25. For more details you can look at our analysis note and you'll 
also have soon access to our long paper which is under the second round 
of Ad Hoc review right now. In it, all the analysis details are 
described.

> First, when you write -t<<Q2, leading twist, it different from -t<Q2 or -t 
> ~O(Q2). So -t/Q2 ~ 0.65 is really big, in my mind,
> and as you pointed, according to Dieter -t/Q2 < 0.25.
> The way I look on this is very simple, through out the paper you talk about 
> "leading twist" that is defined with -t<<Q2. In
> Fig. 4 VGG model, for example, misses data at small Q2 where -t/Q2 ~ 1 to 
> 0.5. So the question is, is this due to
> axial charge or higher twist effects. So, if it is not clear to you, then it 
> , in my mind, misleading for reader. why you are ignoring second effect and 
> stating only the first one. Why not write a sentence or two explaining 
> exactly this.
>
What I meant is that it is not clear to me what exact kinematic limits one 
must take as starting point of higher-twist effects.
But as far as the slope of Htilde (and hence of the axial charge) to be 
what drives the t behavior of the TSA, I am fairly confident. If you look 
at the 6th panel of Fig. 4, where we integrated our data to compare them 
to the world data, you see that the TSA is quite "flat" (apart from the 
low-t drop) for all t, starting from ~0.2 or so (and our "integrated" <Q2> 
is 2.4). What I mean with this is, it is not because of a single point 
at high t that we conclude that the axial charge is more concentrated in 
the center of the nucleon than the electromagnetic one, our TSAs are 
flatter than the BSAs over all the t range.
This statement about the relative slopes of H and Htilde is also confirmed 
by the CFF extraction that we have just carried out using Michel Guidal's 
code, and combining our TSAs, BSAs and DSAs (all from eg1-dvcs, at the 
same kinematics). You'll see this also in our long paper.
Anyway, I'll try to rearrange the text to mention the possible 
higher-twist effects.

I will have a new version ready by the time the collaboration review ends, 
hopefully it will be satisfactory.

Thanks again and best regards,
Silvia