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

[Stepan Stepanyan]

Hello Silvia,

OK, first I do not want make things complicated, I have just a simple 
issues which
I though can be resolved with very simple changes. But, you had your ad 
hoc review
and have the best experts around you, so if they all agree with the 
draft then it is fine.

Never the less I want to make my points more clear -
Yes, you right non of previous papers had "leading twist" in the 
abstract, but non of
previous papers defined asymmetry as a "signature" of the interference. 
In my mind
in other papers it is more careful wording there, less exclusive statement.

You making quick transition from describing the trigger to the off line 
cuts without
telling the "general PRL reader" about it. Two words at the beginning of 
the sentence
can fix the problem "In offline analysis, energy cuts on the EC ...".

I see your distributions, they are very nice and nice connection to 
axial charge you made.
Really good physics outcome. But what I am little bit uneasy about is 
the start and the end
of the paper. You start with making assumptions that leaves only four 
GPDs in leading order
to get the sin(\phi) asymmetry, one of assumptions is -t<<Q2 (whatever 
you think it is) and at
the end without telling the reader how any of these GPD models deal at 
-t~Q2 kinematics,
you just make physics statement because of t-slope and differences with 
GPD models.
At least we could have a sentence or two for large -t/Q2 part of the 
data/comparison.

Regards, Stepan

On 10/8/14, 6:52 AM, Silvia Niccolai wrote:
> 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
>