[Qweak_transverse_prl_comments] Fwd: [Qweak_bnssa_elastic_ep_authors] Final draft of the elastic ep transverse paper - Comments due by October 17th, 2014

[Buddhini Waidyawansa]

---------- Forwarded message ----------
From: Mark L. Pitt <pitt at vt.edu>
Date: Wed, Oct 15, 2014 at 11:57 AM
Subject: Re: [Qweak_bnssa_elastic_ep_authors] Final draft of the elastic ep
transverse paper - Comments due by October 17th, 2014
To: Buddhini Waidyawansa <buddhini at jlab.org>



Hi Buddhini,

Below are my comments on the paper.  You'll see that they are broader than
just minor edits.  I am basically just trying to give you examples of
places in the paper where I think we need to have a broader discussion
about what the collaboration agrees the message should be.  We never had a
formally assigned writing team for this paper (as far as I know), so I
don't have a good sense of how much discussion with others went into the
"message" of this paper.  If there has already been significant discussion,
then I think we need to hear a little more about it to understand why
particular things were emphasized.

First let me say:

* The analysis section in the middle (and the associated tables and
figures) certainly seems consistent with what you presented in your July
endorsement talk.  So I think it is fair to say everybody has had a chance
to see those numbers and give feedback on them even before the paper was
distributed.
* You have clearly put a real effort into making sure the paper is timely
and up to date with all the most recent references and examples of places
where TPE observables come into play.  That is partly what delayed my
comments.  I had to look through all the recent references you have
compiled to try to get up to speed even to make comments.
* We all agree this is a very nice analysis and measurement that you have
put a lot of effort into (both in the analysis and careful documentation of
what you did).  The measurement and the result are a collaboration result,
so it is important to make sure there is a clear consensus in the
collaboration about the "message" that goes along with the result.

The parts of the paper that I am commenting on are those that were not
discussed extensively during endorsement talk discussions.  That is the
"physics motivation" and what explicitly we say about the theoretical
calculations.  Ideally, that is the kind of thing that a writing team
discusses and agrees upon before the primary author sits down and writes
the paper.  If there has already been such discussion, then I would be
happy to hear about it and how it led to this particular message.  We don't
need to re-invent the wheel if there are already solid arguments why this
message is the appropriate one.

Now to comments:

Abstract:  Some of these comments will make more sense after reading my
later stuff below.  In general, I think the abstract needs to have more
details about OUR measurement and how it fits into the existing body of
measurements.  The abstract shouldn't have statements that aren't further
developed in the paper itself.
Specific sentences I am concerned about:
* The MUSE sentence doesn't belong in the abstract.  It is one example
among many of why this general topic is interesting, but it isn't important
enough to be in the abstract.  This is an article describing our
measurement; it isn't a review article.
* The last sentence doesn't seem appropriate either.  It is very broad and
you don't really support it further in the body of the paper (ie. you don't
describe other types of experiments where the target polarization
uncertainty is dominant).  I don't know why this sentence is here.
* What is missing from the abstract:
** In general, more about OUR measurement and where it fits in and the
"message" of this paper
** The actual asymmetry we measure (with its errors) should be in the
abstract
** Make it a bit more clear that this measurement is contributing to a body
of measurements of this quantity, and its special niche is forward angles
** Indicate that we compare our measurement to available model calculations
and the conclusion of that comparison is ? (I'm still trying to figure out
what goes in place of the ? mark; see below).

First part of paper: "physics motivation"
* My general impression is that there is too much space devoted to
observables that depend on the real part of two photon exchange.   I
appreciate that you have taken the time to be very timely and up to date
with all those examples.  But, as you know, we measure the imaginary part
of these amplitudes, and the relation between what we measure and what is
needed for those observables is not completely clear (I know there is a
dispersion relation that relates them).  I think this long paragraph can be
edited so it is just a list of examples of observables where one needs to
know the real part of TPE.  Once again, this is an article about our
measurement - not a review article.  To make it more about our measurement,
I would suggest having a paragraph prior to it which indicates - in general
terms - what BNSSA measurements already exist on the proton and what our
special kinematic niche is.  Make it very clear that we measure the Im part
of TPE.  Indicate that there is in principle a relation between Re and Im,
Then the following paragraph describing all the interesting observables
where Re(TPE) arises seems well motivated.
** Let me give just one example of why I think there is too much space
devoted to the Re(TPE) stuff.  The last part of that paragraph goes on at
length about the TPE corrections in the MUSE measurement, and - given the
amount of ink devoted to it - I think the reader is left with the
impression that our measurement is very important for sorting it out.  But
I think that leaves the wrong impression on several counts:
** It is important to have an estimate of the TPE in the mu-p scattering
measurement for planning purposes, but after all the MUSE folks are not
relying on that.  They will measure all four possible scattering
combinations - so they will measure the TPE effect, not rely on a
theoretical calculation.
** Does good agreement of the Im(TPE) with our (and other results) really
ensure that the same model properly predicts the Re(TPE) effect?  I have
never seen that asserted quite so boldly in the literature as you do here.
I read the Tomalak/Vanerhaeghen paper, and they don't assert anywhere that
BNSSA measurements are needed to benchmark their calculations (and I didn't
see it in Mark's talk at PAVI either).   In fact they don't mention them at
all.  But our paper seems to indicate it is very important.  (Contrast this
to the gamma-Z box situation where even the first Gorchtein/Horowitz paper
made it very clear that PVDIS data would be important for benchmarking
their calculations).

----> I guess my general point about this motivation section is that there
are many comments about the situation that are not appropriate for us to be
making.  Examples are: "would benefit from improved TPE models", "only way
the model prediction can be will constrained", ...  These are conclusions
that we as a collaboration are not qualified to make.   They might be
appropriate for a single (or few) author review article on the TPE topic,
since they represent the opinions of the review article authors.  But the
Qweak collaboration shouldn't be making these kind of statements and
conclusions about that whole field.  Those are the kind of comments and
conclusions that need references to papers by expert authors who people do
trust to make those conclusions.  If those conclusions were stated in a
paper or conference proceeding, then it might be reasonable to include them
with a reference.

Last part of paper: This is an important part of the paper.  We need to -
as a collaboration - come to a consensus on what the message is here.  Many
of the statements made here are bolder than I have seen written in the
literature before and are once again conclusions the Qweak collaboration
cannot make on their own.  I know that much of what is written here was
stated in your thesis, and I assume that it was based on discussions with
theorists.  So maybe it is simply a matter of supporting some of these
statements with references to private communications or conference
proceedings where these statements were made.  The other thing that is
missing here is some notion of how we fit in with the other small angle
data (the two G0 points and the HAPPEX point).  You have some mention of it
(G0), but you don't mention HAPPEX (which also agrees with the Gorchtein
prediction, unlike the G0 points).
Some specific concerns:
** Discussion about the D and F states in Pasquini.  I have heard you state
this before, and it sounded reasonable to me.  But as stated here, I have
some questions.  If the Delta really dominates at our kinematics, then how
come adding in an extra 50% for D and F will fix the problem?  I assume
they are already in there because their dominat decay mode is in fact
single pion.
** Your statement that "our precise measurement confirms with a high
confidence level that single-pion excitations do not ...." is much stronger
than you have stated in talks.  At PAVI you said:  "Our precise measurement
indicates at forward angles, B_n favors models with multiple pion
excitations with the nucleon."  That seems to me a more defendable
statement.  It is not 100% obvious to me that there aren't other
differences between those models that could cause the differences.  Have
all the theorists agreed that this is the main reason for the difference?
If so, then let's have a reference to a conference proceeding or private
communication that supports that claim.
** Do Afanasev and Gorchtein agree with your conclusion about the
difference in their calculations?  I looked at Gorchtein's paper, and I
couldn't find such a clear statement about the difference, but perhaps I
missed it.  If there is a clear statement about this somewhere, then it
needs to be referenced.
** Your last two sentences make pretty bold statements about the importance
of these types of measurements for improving the Re(TPE) estimates for
other observables.  It is much more bold and definitive than I have seen in
the literature before.  Usually just vague, general statements are made
about the connections.  I always assumed that was because nobody has
written a convincing statement about the connections.  Is there a reference
that does that?  If so, quote it here.  For me, I see the vague general
connection, but beyond that I would need to be convinced.  After all the
dispersion integral is over a wide range of kinematics.  Just because we
measure some observable that agrees with a model in a certain kinematic
range doesn't mean that model will give the correct Re(TPE) prediction when
you integrate it over the broad range of kinematics needed to do the
dispersion integral.

Okay, I know that is a lot of comments, but hopefully it gives you a flavor
of my concerns.  I know I haven't given a lot of suggestions of how things
should be modified to address my concerns.  But I can't really do that
until I understand what the collaboration's desired "message" is with this
paper.  Then it is clearer what to write.  I have discussed this with a few
people, and there is some agreement with aspects of what I say.  So I'm not
the only one that feels this way.

In any event, I think there needs to be some sort of phone discussion soon
among a few folks to hear more about your rationale for the message in this
paper.  You have done the most reading and discussion with experts of all
of us, so it could be that your message is the most up to date and
consistent with current thinking.  But we need to understand that, and
since many of the most bold and important statements were unsupported by
references, we need to hear your rationale.

Perhaps the best thing at this point is for you to digest the above, and
then I can discuss it with you on the phone when it is convenient for you.

Regards,
Mark




-- 
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Buddhini Waidyawansa
Postdoctoral Fellow
C122,
12000 Jefferson Ave,
Newport News, VA 23602.
TP 757-912-0410
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