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

[Michel Guidal]

Hello all,

Good work ! Just a few remarks:

p.1, l.15: "More than 50 years after Hofstadter's proof of the non-point-like nature of the proton [1]". Actually, the first evidence of the "non-pointlike" nature of the proton
was, I believe, the experiment of Stern in 1933 who measured the anomalous magnetic moment 
of the proton. I wonder therefore if this very first sentence of the article is not a bit
conroversial...

l.60-63: the Fourier transform is valid at xi=0. TTo be strict, one might want to add 
somewhere in the sentence "at xi=0".

p.2, l.71-73: "Using the invariance of the strong and electromagnetic interactions 
under parity and time reversal,...". This seems to me a little bit far-fetched/pedantic.
One can also invoke Lorentz-invariance, one-photon exchange, etc.. I am not sure
that we need to go that far in terms of justification...

l.78: I am glad that one uses "my" definition for CFFs but be aware that there are
also other notations in the literature ("pi" factors for the "Im"'s, "-" sign
for the "Re",...). Maybe, it is worthwhile a footnote/warning in the text ? (I think
that D. Mueller calls "my" CFFs "sub-CFFs" -which is fine to me-).

l.82-83: I would rather use (quark) "helicity-(in)dependent" rather than "spin-dependent"
since it refers to quarks rather than the nucleon. 

l.85: "different sensitivities to the four GPDs can be realized". Just vocabulary:
"realize a sensitivity" sounds a bit weird to me. 

l.88: the CFF "Im Htilde" of the GPD Htilde" sounds a bit redundant to me.

l.88: "Conversely". The word means to me that there is an "opposition"
(while there is not). Wouldn't "Similarly" or something like that be better suited rt

l.94: better than "particle", I would use "current".

l.95: "providing a bridge between the strong and the weak interactions". Sounds to me
a bit far-fetched/pedantic again. The standard model provides the bridge, I don't 
think that we need go that far to explain what are H and Htilde.

l.102: "imaginary CFFs". Be careful: if you use "my" notation of the CFF, the CFFs
are "real". Better speak of the "Im CFFs", which are well-defined in the bottom 
part of Eq.1 and which are "real".

l.104: "magnify" seems too strong to me. Something like "boosts", "favors", "enhances",... ?

l.111 "low-statistics". "Low" is a relative notion. I think that it is a bit "snobby/patronizing" to qualify the HERMES measurement of "low statistics" as 
they have been running for years and that their statistics is not that low. 
Maybe "limited statistics" ? The rematk also applies to l.123.

l.247: since Eq.2 is mentioned, I wouldn't erpeat the form of the
function that is used for the fit: "the function of Eq.2".

l.257-263: "The trend...". The sentence is pretty long: 7 lines! Not possible
to split it ?

l.263: "It must be recalled". Why "must" ? "We recall that..." ?

l.267: if one wants to be really precise, one can/should add something
like "via a Fourier-like transform... and modulo a "deskewing" 
(GPD(xi,xi,t)->GPD(xi,0,t)) correction"...

l.272-276: 6-lines long sentence. Possible to split ?

l.330: "For the first time four-dimensional target-spin asymmetries 
with longitudinally polarized protons...". It might be controversial.
HERMES and CLAS (with Shifeng) has also measured "four-dimensional"
asymmetries. The difference is that they have only one four-dimensional bin ! 
Maybe there is a better way to phrase things ?
 
l.349: "to improve the parameterizations of the GPD Htilde".
Maybe a bit of caution here: H is also participating to A_UL
and there is the possibility that Htilde is correctly parametrized
but not H... Only a joint analysis of AUL and ALU can answer (which
might be beyond the scope the paper)


Good luck !

Michel