[Clascomment] OPT-IN: gamma n --> pi- p Differential Cross Section Measurements with CLAS

[Reinhard Schumacher]

Hello Paul et al.,

It is very nice to see this g13 result ready to see the light of day.
I have read the paper in some detail and I have a lot of stylistic
comments and just a few substantive questions about the text.

Title: I don't think we need to trumpet that the measurement was made
at CLAS, given that it gets mentioned in the first line of the
Abstract.  It may also not be wise to start the title with a
Greek-character string, since that will forever confuse the indexing
and citation of this paper.  How about "Differential Cross Section
Measurements of $\gamma p \to \pi^- p$ Above the First Nucleon
Resonance Region".

The Abstract I think is a good summary of the paper.

PACS numbers are no longer used by the publisher of Physical Review
and can be left off.

Table 1 caption:  I don't think the "Delta" is defined anywhere.

line 85:  "...at THEIR pole on the..."

In my view the next few sections of the paper suffer from a case of
TMI ("too much information") in several places.  What any paper has to
do is convince the reader that the scientific results are reliable.
It is possible to put in too much minutiae, which actually detracts
from this message.  When it comes to descriptions of CLAS, we have
well over 100 papers published and many of them rehash the same
information about the detector system.  I would try to trim down these
sections in the present paper to concentrate only on what is unique to
the g13 data set.

>From this point of view, Figures 1 and 2 and (maybe) 3 and 5 and 7 and
(probably 9) are superfluous or distracting from the main message.
Either they duplicate what has been shown many times before or are too
prosaic to teach the reader anything useful.

Since you cite the standard CLAS references for the detector and
tagger, you can probably streamline the presentation by removing or
drastically shortening:

lines 113 to 126
lines 130 to 141
lines 148 to 160
lines 148 to 210
lines 238 to 258

Figure 5 I find especially unfortunate.  What is the message the
reader here?  He or she is not going to be able to form any judgement
about the quality of the experiment from this: are you trying to say
the detector is full of holes (which is true but not a problem)?  Is
the message that figure represents is the irreducible limit on the
precision of the experiment (and if so, how does this work out in the
end?)?  In short, I think this figure should remain internal to us
CLAS mavens, in the Analysis Note, and not part of the final published
product.

lines 309 to 320 I believe are another example of TMI.

Fig 7 caption: IF you keep this figure (which I think is not needed),
then "...Sectors 2 AS A FUNCTION of momentum p...", "...triggering was
OVER XX% efficient...", "...efficiency was (REMOVE 'much') lower..."

Fig 8 caption: "Sample fit FOR the missing-HYPHEN-proton peak..."

line 350: seems like you are pointing to the wrong paper with Ref 8.

line 374:  replace 'large' with 'hard to quantify'

Fig 9 caption: IF you keep this figure (which I think is TMI in this
paper), use "The CALCULATED CLAS acceptance.... angle (REMOVE 'in
terms of')..."

line 433: "...coincident-HYPHEN-matched..."

line 530: IN the discussion of the comparison of CLAS results with the
other experiments, you mention that they give normalization
uncertainties of about 5%.  But you should also look up and quote what
their respected total systematic uncertainties are.  They could be
bigger and help explain the observed discrepancies.

Equation 6:   awkward typesetting in the last term.  Try using
"absolute value" bars around vector p_s.

Fig 10 and 11: Splendid results!  I would clean up the x-axis labels
by removing the parenthesis around the angle in cos\theta^cm_\pi.
They only clutter the pictures.

Fig 12 caption:   use "...pi^- p SHOWING CROSS SECTIONS vs. W..."

lines 636 to 638): it is not clear to me what you are trying to say
here.  It is a convoluted construction if you are only trying to say
that R varied from 70% to 90%.

lines 666 to 670: I am concerned that here you introduce "quasi-data"
using the SAID model that you then fit in the Legendre analysis.  Why
is this legitimate?  A critic will say that it is not permissible to
dream up a model that you HOPE is right and then fit the mixed real
and fake "data" to present an analysis of the real data.  You fit up
to J=10, but you don't address the reliability of extracting
coefficients of angular momentum this high.

line 702:  use "...in the MAINZ A2..."

Table II caption:  the 3rd column heading is disturbing because it is
unclear what it means.   Presumably you want to say "/chi^2 / data
point" or "\chi^2/d.o.f."

Fig 15: The storm of redundant red arrows weakens the impact of this
figure.  I suggest putting arrows for each resonance only once on the
figure, maybe in the top on in the third row.  Also, in the caption,
"...by nxAj+m FOR EASY visualization".

line 844: Here you mention the N(1720) twice in a row, which is sort
of silly in this context.

line 943:   I think the ending of the paper reads very well.

That's all for this round of comments, let me know if you have any
comments.

Best Regards,
Reinhard