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

[Volker Burkert]

Dear lead authors. 

It is good to see that this letter is now in the final stage of review.  It is quite well written and I have 
only a few comments. However, I have not checked if my comments overlap with previous comments, 
so there might be repetition, and since Silvia responds immediately to everyone making comments 
it becomes unclear what the actual status of the paper is. So, I ignore previous responses as well. 

Volker


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. 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.    

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". 

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.."

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" . 

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. )

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 ...."  

Line 91, 92,93: "electromagnetic charge" => "electromagnetic current",  "axial charge" => "axial current".  

Line 145: "nearly 4\pi acceptance" is a stretch with all the polarized target coils in the way.  
               "large acceptance" is better.

Line 156: ".. for time-of-flight identification" => "for time-of-flight measurements and particle id". 

Line 171: "to select DVCS events.." => "..to select single \gamma events..". 

Line 173,176: "..black shaded plots.." => "black shaded areas.."  

Line 269-271: same comments as before. "axial charge" => "axial current", "electromagnetic charge" 
                       => "electromagnetic current". 

Line 322: "using low-x_B HERA data" give x-value => "using HERA data at very low x < 10^-4 ??)"