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

[burkert]

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
>