[Clascomment] OPT-IN: Photoproduction of pi0 on Hydrogen using e+e-(gamma) detection mode with CLAS

[Reinhard Schumacher]

11-27-2017

Hello Moskov et al.,

I have looked over your draft CLAS paper "Photoproduction of pi0 on
Hydrogen using e+e-(g) detection mode with CLAS" undated, but posted
around 11-20-2017.  I understand that the target journal is Physical
Review Letters.

I hardly know where to begin in making comments on this paper because
it is in such rough shape.  That is, the style and structure of the
paper are poor, there are an enormous number of inconsistencies and
mistakes in the presentation, and most problematic: you have no
substantial physics case to make.  All these things may be fixable,
but in its present state my judgement is that you have zero chance of
getting this into PRL.  Frankly, I am shocked that the Ad Hoc
committee agreed to let this draft go out to the CLAS Collaboration.

I will give you a sampling of comments that can be made about the
paper, but there are many more things that will not be mentioned.

Title: BAD!  It does not state clearly what the physics result(s) of
the paper are.  Who would want to read this paper, especially in PRL?
You must make a strong case.  How about "Elementary photoproduction of
pi0 at high energy and high t"?  The fact that you used Dalitz decays
is incidental to the measurement results.  But even my suggested title
is weak because it has no punch.  What IS the main conclusion of your
work?

Abstract: It does not make a strong case for your work.  The last
sentence is a hint of why your work is interesting, but the
"quadruple" detail is much less important.  You have a weird notation
here and elsewhere where you write "\gamma p \to p e+ e- X(\gamma)";
the "X" conveys nothing and ought to be removed.  The following
sentence to that phrase is entirely redundant.

The paper begs for some additional structuring, apart from some
reorganization.  I suggest section headings:
line 7: remove "Abstract"
line 9: insert "Introduction"
line 122: insert "CLAS Measurement"
line 227: insert "Results"
line 281: insert "Summary"

The first page of the present draft is of interest only to people who
know the field very well.  To get a paper into PRL you ought to have a
more general introduction that everyone can at least partly
understand.

By the end of the first column of the paper the reader should
understand WHAT you did, and WHY it advances the field.  You can
outline Regge vs. counting rules vs. handbag models, but the details
ought to go later, after you have presented your results.  The
detailed discussion can then be made in light of your new results.
The present organization leaves the reader with little mooring to your
(eventual) scientific message.

line 124, 129: your notation for the cross section is idiosyncratic.
Why write "(t)s".  This is unnecessarily obscure.  Just say
"d\sigma/dt as a function of t" (or whatever you are trying to say
here).

line 156: It may not be OK to refer the reader to an unpublished
document to describe the experimental details.  The standard thing to
do is to refer to a long/archival paper that explains everything, even
if that paper is given as "to be published".  I assume g12 will write
such a paper.

line 183: You talk of cuts on C.L. and tuning thereof, but then don't
tell the reader what the pro's and con's of your choices were.  I
think you need to spell this out fully... or delete this sentence if
it is not important.

Fig 1 caption: I am quite confused by this figure.  First of all, you
should specify the axes as "Y versus X": your caption has it
backwards... twice.  I am also not sure what the vertical axis on the
left side is.  The missing mass off the detected proton, e+ and e-
should be the photon, not something centered at 0.3 GeV.  For the
right-hand panels you have blobs that are centered on zero for the
vertical axis, which I interpret as the missing photons, but the label
in the figure say pi0.  Also, how do M_E and M^2_x differ?

line 226: again you commit the sin of referring the reader to an
unpublished report.  The reader has no idea, for example, what you
mean by "sector to sector" systematic uncertainty.  This should be
spelled out clearly.

Figure 3 and Fig 3 caption: First, there is wasted space here and the
figure is anyway too small to interpret quantitatively.  Use the extra
space.  Maybe show only ONE BIG PANEL with the data offset for the
different energies.  Then the reader would have a hope to see the many
lines and points clearly.  You refer to "tagged" and "bremsstrahlung"
data here (and elsewhere), but this is an irrelevant distraction.
Yes, CLAS used tagged photons and yes, old experiments used untagged
bremsstrahlung, but so what?  That is probably not relevant.  As long
as you trust the old data, just plot it without this detail mentioned.
On the fourth line you use "open filled", which makes no sense.  You
refer to [26] which is, however, not a SAID paper.

line 254: You point out a dip at |t|~5 GeV^2.  There is no dip in the
data at that momentum transfer.  Do you mean 0.5 GeV^2.

line 260, 261: Here again you refer to "possible new structure" around
5 GeV^2.  This is not what your data shows.  Why the scare quotes
around that phrase?

Figure 4 caption: here is another instance of using odd notation for
the cross section.  You don't need to include "(|t|)".

line 302: Michael Kunkel should get this thesis link given in the
references for the paper.  That is common CLAS practice.  In the
references put the link to
https://www.jlab.org/Hall-B/general/clas_thesis.html .

That's all for this round.
Cheers,
Reinhard