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

[Stepan Stepanyan]

Hi Silvia,

Thanks for the email and for considering most of my comments.
But I still have issues with remaining. Below my replies to yours.

On 10/7/14, 10:06 AM, Silvia Niccolai wrote:
> Hello Stepan,
> thank you a lot for your comments. Here are our replies to some of the 
> points you raise:
>
>> abstract - phrase "..., which are a signature of the interference of 
>> the deeply virtual Compton
>> scattering and the Bethe-Heitler processes, ..." strictly speaking 
>> this is not true, in DVCS amplitude there is a term
>> $s^{DVCS}_{1,LP}$ that can generate sin(\phi) modulation.
>
> Looking at Belitsky's paper, the s^{DVCS}_{1,LP} term you mention is 
> the one coming from the DVCS^2 contribution.
> This term appears with a K (~t/Q2(1-xB)...) coefficient, which means 
> that this should be suppressed at leading twist. In the Belitsky 
> paper, the s^{DVCS}_{1,LP} term is indeed said to appear only from 
> twist three.
> We think that for the abstract limiting ourselves to discuss the main 
> sin(phi) component, which is the one due to the interference term, is 
> appropriate. Otherwise, how else would you propose to phrase it?
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, ..."
>> line 38 - sentence make no sense, should be revised. What it means 
>> "... considering only quark GPDs ..., there are four different GPDs ...
>
> It means neglecting gluon GPDs. Usually the typical sentence is 
> "considering only the quark sector". We changed it this way to make it 
> more understandable for a general audience, following a request of our 
> Ad Hoc committee. Is it really not clear? Why do you say it makes no 
> sense?
OK, sorry I said makes no sense, it just looks like to many overlapping 
assumptions.

>> line 152 - at the trigger level CLAS cannot discriminate between 
>> negatively and positively charged pions, so remove word "negative" in 
>> front of "pion"
>
> Actually, here we are no longer at the trigger level, we are referring 
> to the offline PID cuts on the EC energy deposition, which are done on 
> negative tracks (based upon the DC information). So we think it is 
> correct to leave the word "negative" in front of pions.
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?
>> line 266, line 340 - discussions of t-slope and axial charge, while 
>> it clearly there will be
> connection between t-slope, spatial position towards center and the 
> axial charge, it is not clear
> to me how large t/Q2 plays here for the presented simplistic model at 
> leading twist/leading order.
> Paragraph on page 1, line 45, clearly states that this process is 
> valid for t/Q2 << 1, is not it.
> Some explanations are needed in addition to the axial charge 
> distribution.
>
> Yes, it is true that we are being a bit "general" here (but being it a 
> PRL, we have to), and not going very deeply into discussions that go 
> beyond the leading-twist approximation.
> Let me first say that the data we select have the cut -t<Q2, so no 
> events have t greater than Q2. If you look at Fig. 4, you'll see that 
> our worse t/Q2 ratio is for the last point of bin 1, and it is ~0.65. 
> Where exactly is the limit between leading-twist and twist-3 in terms 
> of t/Q2 is not fully clear to me. According to Dieter Muller this 
> limit is -t<Q2/4 (and you see it in the blue curves in this same 
> figure). According to others, Dieter is too "strict". So, you say that 
> it is not clear to you how large t/Q2 plays a role here, well, it is 
> not clear to me either...
> Our data have low precision in the low-Q2/high-t bins, so 
> unfortunately they don't allow us a very refined analysis extracting 
> the various sin(nphi) terms, to look for higher-twist effects. In our 
> long paper, which is under Ad Hoc review at the moment, we also 
> include the beam-spin and double-spin asymmetries, and this allow us 
> to constrain the denominator cos(phi) term, and to make a little 
> exploratory study adding a
> sin(2phi) term to the numerator of our fitting function, but even so 
> the precision of our data doesn't allow us to say anything conclusive 
> or strong enough as far as where the leading-twist ceases to be a 
> valid approximation. So we can't really say much about this, in this 
> paper.
> As far as our statement on the t-slope of the asymmetry being flatter 
> than that of the BSA, I don't think this would be modified if we went 
> beyond the leading-twist approximation. What would change would be 
> what combination of GPDs would be involved in this observable 
> (non-linear combinations of GPDs would come into play). But I would 
> dare say that the higher-twist corrections wouldn't be so important to 
> change the fact that Htilde is the "dominant" GPD in the TSA.
> Bottom line, the issue of the higher-twist effects is quite 
> complicate, and I am not sure this paper, and these data, are going to 
> bring much light into it.
> If you have any suggestions on how to address this aspect that you 
> raised, please let us know.
>
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.

Regards, Stepan