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

[Michel Garçon]

Nice results and paper. Here are some comments for your consideration:

- In the abstract and in some instances in the text, the TSA are qualified as a signature of the DVCS/BH interference. Strictly speaking, any process which conserves helicity will, when interfering with BH, generate a sin(phi) spin asymmetry. In addition, DVCS2 can also generate a sin(phi) term at leading twist: see BMJ, arXiv:1212.6674, eqs 41 and 49. What is true is that within all GPD models, BSA and TSA arise mostly from DVCS/BH interference. So instead of "which are a signature of the interference...", something like "which are expected to arise mostly from the interference".

- Abstract last line: chiral-even GPDs.

_ line 47: Q2 = -(e-e')2

- line 56: remove "exactly", which is antinomic with perturbative (in a perturbative expansion, there is always a truncation).

- line 67-68: "The BH process has a larger contribution to the cross section than DVCS in most of the phase space, but ..."

- line 76: statement is not correct (see first comment).

- line 78: the Compton form factors were introduced earlier, e.g. in ref[6]. The alternate definition in Ref [8] leads to ambiguities, like in line 101-102: there you mean the four ImF (in Ref[8] definition), which are not imaginary !! The 4 CFFs in [6] are imaginary, the 8 in [8] are real. I would strongly urge that we use the definition of [6], and then we can talk unambiguously of their real and imaginary parts.

- line 80: "a "generic" GPD" -> "any of the four GPDs"

- line 82-83: more precise to say "quark-helicity (in)dependent" than "spin (in)dependent".

- line 94: subscript in a1.

- line 98: should not there be a Q2 dependence as well ?

- line 110: see first comment

- line 118: can safely remove "DVCS/BH" here.

- line 157-158: latex for degrees: $^{\circ}$.

- line 171 and 203: replace DVCS by e'p'gamma

- line 217 (optional): strictly speaking, it would be better to state that this (small!) correction is model-dependent

- line 234: 15% instead of 14.8. (hard to calim such a precision on the systematic uncertainty!)

- line 241: see first comment

- line 264-265: the statement that TSA is mostly sensitive to H_tilde may not be correct. The main (and comparable) contributions to TSA come from both H and H_tilde. You could illustrate this by plotting in fig.3 either H only or H_tilde only contributions (not necessarily for the paper, but to verify my statement). I think this must be made clear in the paper. BSA is mostly sensitive to H, while TSA has an added sensitivity to H_tilde. Then this will temper the conclusions about the t-slope uniquely attributed to H_tilde.

- Fig 4 caption: HERMES uses a moment (which I do not like because of the denominator); it should be stated at the end of the first sentence that it is not exactly the same quantity which is plotted: after "sin(phi) amplitude of AUL", add in paranthesis "(alpha, or AUL(90°), or sin(phi) moment of AUL depending on the publication, all definitions giving the same results within the present uncertainties)".

- line 332: arising mostly from

- line 347-349: the statement is too strong, first because not only H_tilde contributes to TSA, second because GPD models at leading twist are not expected to be valid when t/Q2 is not small.

- ref [6]: consider adding (within the same reference): BMJ, arXiv:1212.6674 (I do not know why it is not published...); BMK did not give the formulae for TSA.

- ref[17]: add in the same reference which parameterization of the FF is used.

- ref[20]: I believe it is published in EPJC 53, 349 (2008). Please check it is the same.

That's all!
Michel Garçon.