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

[Silvia Niccolai]

Hello Volker,
you are right, electromagnetic charge is wrong, I modified the paper 
changing it into "electric charge". Thank you for pointing this out and 
my apologies for the inaccurate wording.

On the other hand, I looked into the literature and discussed with Michel 
at length and I don't see an actual mistake in keeping the expression 
"spatial distribution of the axial charge". The spatial distribution of 
the axial charge is linked to the Fourier transform of the axial form 
factor - which in turn is the first moment in x of Htilde. So if you don't 
object too strongly I would keep this wording, to be consistent with 
what is said in the papers we refer to in the PRL when dealing with this 
subject (such as  Eur. Phys. J.C 53, 349 (2008)).

Thank you again for your comments.
Best regards,
Silvia


On Thu, 16 Oct 2014, burkert wrote:

> HI Silvia,
>
> Just one comment in reply.
> "Axial current" is NOT jargon. A famous theorem in particle physics is PCAC, 
> i.e. "partial conservation of axial current". You also insist on using the 
> term "electromagnetic charge" . There is no such thing. There is the 
> "electric charge" and "magnetization". The best way to describe both in 
> general terms (and used in all theoretical papers) is to define it as 
> "electromagnetic current" and for the Fourier transform in t "distribution of 
> the electromagnetic current in transverse space".
>
> Cheers,
> Volker
>
>
>
>
> 10/16/14 8:38 AM, Silvia Niccolai wrote:
>> Hello Volker,
>> thanks for your remarks. I am sorry if my way to respond directly to each 
>> collaborator commenting appears confusing to you. I have been trying to 
>> address all potentially "controversial" issues right away, discussing with 
>> whoever raised issues. At the same time, I have been keeping track of all 
>> comments on which there was mutual agreement, including them in a new draft 
>> that I shall send out tonight to the Ad Hoc for final approval. I didn't 
>> post modified intermediate versions during collaboration review to avoid 
>> confusions.
>> 
>> Here are my replies to your comments:
>> 
>> On Wed, 15 Oct 2014, Volker Burkert wrote:
>> 
>>> Abstract:
>>> 
>>> 1)  " ....  target-spin asymmetries ....a signature of interference of the 
>>> dvcs and Bethe-Heitler processes.."
>>> is misleading the way it is written with the double polarization process 
>>> e-pol p-pol -> epg. Bethe-Heitler
>>> alone produces the largest asymmetry in the double polarization process. 
>>> I suggest to remove the
>>> polarization arrows over e and p.
>> 
>> OK, done.
>> 
>>> Also the "for the first time" is a stretch with both CLAS and HERMES
>>> having published longitudinal target asymmetry data before, unless the 
>>> "first" is meant for the "166 bins
>>> " or the "in a wide kinematics".  It might be better to say that this 
>>> experiment has accumulated target
>>> spin asymmetries in the widest kinematics in Q^2, -t, x_B, and phi.
>> 
>> The "For the first time" refers to the fact of having:
>> 1) four-dimensional bins, while Shifeng and HERMES integrated over 3 
>> variables at a time
>> 2) many of them
>> 3) wide coverage
>> 
>>> 2) "..provide insight on the spatial distribution of the axial charge of 
>>> the proton, which appears to be
>>> focused in its center."  As for the electric charge the "axial charge" is 
>>> only defined at -t=0.
>>> For non-zero t, we should speak of "axial current". It is also better to 
>>> write " concentrated" rather than
>>> "focused".
>> 
>> Following your comment, I discussed your remark with Michel Guidal, as he 
>> had originally made this statement about the slope of Htilde and the 
>> distribution of the axial charge in one of his papers of CFF fits. In his 
>> papers, he normally refers to the spatial distribution of the axial charge, 
>> same as we do in this paper. What he told me, related to your comment, is 
>> that it is true that the TOTAL charge (E.M. or axial) is defined for t=0, 
>> but what we talk about here is its spatial distribution, with the spatial 
>> information coming from the Fourier-conjugate variable of t. So, he doesn't 
>> see any mistake in referring to the spatial distribution of the axial 
>> charge, while on the other hand he fears that referring to axial current 
>> would be too much "jargon" for a PRL.
>> 
>> OK for "concentrated", abstract changed.
>> 
>>> Text:
>>> 
>>> Line 15: Hofstadter's measurement was published in 1956, i.e. we are 
>>> nearly 60 years later (rather than over 50)
>>>             "Hofstadter's proof.."  => "... after Hofstadter directly 
>>> measured the finite size of the proton.."
>> 
>> OK.
>> 
>>> Line 66,67: "At the cross section level BH is "typically" more 
>>> important.." => "At the cross section level
>>>              Bethe-Heitler dominates DVCS in kinematics where the emitted 
>>> photon is close to the incoming
>>>               or the scattered electron" .
>> 
>> This sentence has already been modified following other collaborator's 
>> comments.
>> 
>>> Fig 1: "x is not accessible experimentally in the DVCS process" => " 
>>> Measurement of the DVCS process
>>>           is kinematically constrained to x=\xi".   (VB comment: this is 
>>> similar to DIS which is constrained
>>>           to x=x_B. )
>> 
>> I don't fully agree, as we can also access integrals of GPDs over x, when
>> measuring DVCS observables sensitive to the real part of the amplitude, 
>> such as the cross section of the DSA. So I'd leave this sentence as it is.
>> 
>>> Line 75,76:  " .. depends only on DVCS/BH interference" is misleading. 
>>> What about the denominator in
>>>            the asymmetry? Maybe better " .. the helicity-dependent cross 
>>> section difference depends only ...."
>> 
>> OK.
>> 
>>> Line 91, 92,93: "electromagnetic charge" => "electromagnetic current", 
>>> "axial charge" => "axial current".
>> 
>> Same comment as above.
>> 
>>> Line 145: "nearly 4\pi acceptance" is a stretch with all the polarized 
>>> target coils in the way.
>>>               "large acceptance" is better.
>> 
>> OK, I remove "nearly 4\pi" and put "wide", as we say already "Large" the 
>> line above when spelling out the CLAS acronym.
>> 
>>> Line 156: ".. for time-of-flight identification" => "for time-of-flight 
>>> measurements and particle id".
>> 
>> OK
>> 
>>> Line 171: "to select DVCS events.." => "..to select single \gamma 
>>> events..".
>> 
>> Already modified due to previous comments.
>> 
>>> Line 173,176: "..black shaded plots.." => "black shaded areas.."
>> OK
>> 
>>> Line 269-271: same comments as before. "axial charge" => "axial current", 
>>> "electromagnetic charge"
>>>                       => "electromagnetic current".
>> 
>> See reply above.
>> 
>>> Line 322: "using low-x_B HERA data" give x-value => "using HERA data at 
>>> very low x < 10^-4 ??)"
>>> 
>> 
>> I would rather not specify an x value because in their paper they say they 
>> used data from various HERA experiments (H1, ZEUS and HERMES), which had 
>> various values for x. This sentence was suggested to me from Franck 
>> Sabatie, who worked on this model with Peter Kroll, and I don't think it 
>> needs to be more specific than it is.
>> 
>> Best regards,
>> Silvia
>> 
>