[Clascomment] OPT-IN: Photon beam asymmetry Sigma for eta and eta' photoproduction from the proton

[Reinhard Schumacher]

1-20-2017

Hello Barry et al.,

I have looked over your draft CLAS paper "Photon beam asymmetry Sigma
for eta and eta '... proton" dated 12-30-2016. Congratulations on
getting this important observable measured at CLAS for these two
mesons.  Here are my comments on the paper.

I wish you would have posted this draft with line numbers to make it easier
to specify locations.

Abstract:   I think the last sentence can be more assertive.
Consider  ""...are presented, SHOWING how these new measurements CAN
refine models of the photoproduction process."

page 2, para 1:  Sandorfi published a paper recently showing that simply
measuring a subset of 8 is not enough.   Mathematically it might
suffice, but any measurements have uncertainties, making the
mathematical minimum insufficient to pin down the amplitudes without
considerable ambiguity, including some discrete ambiguities.   Dave
Ireland has also studied this.   You might want to recast this
paragraph to say that we need as much experimental data as possible to
approach the goal of unambiguous determination of amplitudes.

page 2, para 2: surely you don't want to say "relative to a plane
parallel to the floor in the laboratory frame".  Is this statement
just plain wrong?  You want to say "relative to the plane of photon
linear polarization".

page 2, para 2 bottom, "...be important near THE ETA' threshold..."

page 2, top right.  Your paper does not cite the CLAS g8 publication
from Glasgow about all the strangeness-related channels.  I refer the
C. Paterson et al paper from last year.  This is a major omission and
must be fixed.

page 2, right side, below middle:  use ...(CLAS) [27], composed..."

page 3, line 1 "...the degrees of BEAM polarization..." seems to read
better

page 3, below Eq 4:  no need for parentheses around beam energies,
just put them in line with commas

page 3 lower down:  I think removing "GPID" would be good.  This is
CLAS jargon and gets used no where else.  The reader does not need to
see this jargon word.

page 3, right side:  You have a habit in this paper of giving
qualitative statements about the quality of fits, cuts, and
resolutions.  Without trying harder to be quantitative, the paper is
unnecessarily weak.   For instance here you say "...yielded good
momentum definition for proton..."   What is the reader supposed to do
with that statement?   What does "good" mean in this case?   Try to
make a quantitative statement.  Or perhaps remove it.

page 3, Eq (5):   I don't think you need this equation.   It conveys
nothing useful that is not conveyed by the words that you calculated
the missing mass off the detected proton and the initial state.

page 3, right side, lower down:  here you take up a paragraph talking
about detecting the pions and say that the "background was greatly
reduced".   This means very little to the reader unless you show what
you mean quantitatively.   Maybe put the "before" histogram on Fig 1 ?
Alternatively, get rid of the adjective "greatly".

page 3, right side near bottom: The is a very awkward construction
beginning "With this approach..."  In fact you DO bin in Egamma and
cos \theta , and your point is that you do not have to bin in phi.
Why not say that directly instead of starting the sentence with a
negative statement of what you did not do.

page 4, left side middle:  In many places in the text and in the
figures you put parentheses around the \theta in cos \theta.   There
is no need for the parentheses.   The text and figures look better
without them.

page 4 left side, near middle:  tell the reader (and me) why you
picked "weird" bin sizes 27 and 54 MeV instead of rounding to a nearly
round number like 30 and 55 MeV.   Your argument about "best" W
resolution and "good" CM angle resolution is weak.  Can you make it
quantitative? 

page 5, Fig. 2:   I think you can improve the appearance of this
important figure.  Center the axis labels;   remove the parentheses on
cos(theta);  make the symbols a little smaller and sharper, so it does
not look like a crayon drawing.  In the caption you have a "." where
you want a " , ".

page 6, fig 6: Same comment as above about making the figure look more
professional.  I also worry about the scatter of the CLAS data points,
especially in the top right panel.  Is the structure real?  What did
the technical review have to say about this?

Also, in Fig 6 you have the problem that there are many curves.   I am
not sure I correctly figured out which was which.   I would try to
improve the layout.

page 6, right side near bottom.  You make contradictory statements, or
at least phrase things poorly.  On the one hand you say the couplings
changed "noticeably".  On the other hand you point out correctly that
the curves including the extra state hardly change at all.  The reader
might concluded that you are not learning anything new with this data.
Can you be more quantitive about what you mean by "noticeably"? Is it
a 2-sigma change?  Is it a 1-sigma change?  If it is 1-sigma, it may
be "noticeable" but not "significant", for instance.  (In the
concluding section you claim the state parameters where changed
"significantly") If I were the reviewer, I would reject the paper
until you clarify this claim.

References: I like having the titles of papers included in the
references, and I would be tempted to include them in the version of
the paper put on the arXiv.  (I have done that with CLAS papers I put
on the arXiv.)  But, for sure, Physical Review will remove the titles.
On the other hand, I would remove the arXiv references immediately for
papers that are already published.

That's all for this round.
Cheers,
Reinhard