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

[Silvia Niccolai]

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had originally made this statement about the slope of Htilde and the 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