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

[Silvia Niccolai]

Bonjour Michel,
thanks a lot for your comments. Here are our replies:

On Thu, 9 Oct 2014, Michel Guidal wrote:

> 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...

Uhm. So far it didn't raise controversies among the rest of the collaboration. 
I propose to wait and see what the PRL referees think. This experiment is 
the one usually referred to as proof of the non-pointlike nature of the 
proton.

> 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".

OK, added.

> 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...

In one of our initial drafts, we were saying something like "the 
interference of DVCS and Bethe-Heitler gives rise to spin asymmetries". 
Our Ad Hoc committee found this sentence non-motivated enough. Their 
remark was that a PRL should be geared towards a general audience and 
thus all statements should be motivated and explained. 
I looked and found a paper by Markus Diehl in which he used these 
motivations to explain the appearance DVCS/BH asymmetry, so we adopted 
them. 
At this stage, if for you it's not too annoying I'd propose to wait and 
see what the PRL referees think about this point too, or else you can 
discuss it with our Ad Hoc committee. It is really difficult to please 
everyone... :-)

> 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-).

OK, as others complaned about this, I propose the following wording:
"...Compton Form Factors (CFFs), for which we adopt here the definition of 
\cite{mick_herve}...". As the choice of this definition does not impact 
the conclusions of this paper, I would not dwell more into the various 
definitions that are available on the market.

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

Already changed after similar remark by another collaborator.

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

It was suggested by the Ad Hoc committee.

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

Very good, we remove "of the GPD Htilde" and gain some space :-)

> 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

There is a bit of opposition as we want to emphasize the different kinds 
of sensitivities of the two observables.

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

I took this sentence from you ;-)

> 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.

It was requested by our Ad Hoc committee to be a bit "interdisciplinary" 
and to put in evidence some general aspects of the axial charge.

> 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".

This has been changed in the new version, following another collaborator's 
remark. The words "real" and "imaginary" no longer appear in the article.

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

I actually like this word...OK, I'll put enhance.

> 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.

The paper we refer to in this case is Hermes's BSA paper (2001) where they 
had, same as for Stepan's paper, one single phi distribution integrated 
over all kinematics. I would say that for these two experiments there is 
nothing snobby about saying they had low statistics.

> 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".

Good point, and it helps us saving space! :-)

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

It is one of those sentences that went through dozens of modifications in 
the Ad Hoc phase. I'll try to work on it a bit more.

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

As a matter of style, I tend to prefer the impersonal form to the use of 
"we". I'll try to find an alternative form that sounds less "imposing".

> 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"...

I remember that you made this remark to me at the time of our first draft, 
and we had agreed that it was better to leave such technical details out 
of a PRL.

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

I'll try.

> 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 ?

They integrated on three variables at a time whenever they made a t, or xB 
or Q2 distribution. What we say here, that we are the first ones making "4 
dimensional" asymmetries, is the truth. I don't think it is controversial. 
We must emphasize what our data bring out more than what was done before, 
and this is one of the main aspects.

> 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)

Once again, the "new" information that our data bring is on Htilde, as 
data focused on H (BSAs, cross sections) already exist. We are not saying 
anything wrong nor too strong here, the publication of our data 
is important to improve the parametrizations of Htilde. If they help for 
H as well, even better. 
As far as the joint analysis, we say in the final sentence that our data 
will be combined with BSAs and DSAs to extract the CFFs.

> Good luck !

Merci!
Silvia