<div dir="ltr"><br><div class="gmail_quote">---------- Forwarded message ----------<br>From: <b class="gmail_sendername">Mark L. Pitt</b> <span dir="ltr"><<a href="mailto:pitt@vt.edu">pitt@vt.edu</a>></span><br>Date: Wed, Oct 15, 2014 at 11:57 AM<br>Subject: Re: [Qweak_bnssa_elastic_ep_authors] Final draft of the elastic ep transverse paper - Comments due by October 17th, 2014<br>To: Buddhini Waidyawansa <<a href="mailto:buddhini@jlab.org">buddhini@jlab.org</a>><br><br><br>
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Hi Buddhini,<br>
<br>
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.<br>
<br>
First let me say:<br>
<br>
* 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.<br>
* 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. <br>
* 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. <br>
<br>
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.<br>
<br>
Now to comments:<br>
<br>
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. <br>
Specific sentences I am concerned about:<br>
* 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.<br>
* 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.<br>
* What is missing from the abstract:<br>
** In general, more about OUR measurement and where it fits in and
the "message" of this paper<br>
** The actual asymmetry we measure (with its errors) should be in
the abstract<br>
** 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<br>
** 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).<br>
<br>
First part of paper: "physics motivation"<br>
* 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.<br>
** 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:<br>
** 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.<br>
** 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).<br>
<br>
----> 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.<br>
<br>
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). <br>
Some specific concerns:<br>
** 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. <br>
** 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.<br>
** 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.<br>
** 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. <br>
<br>
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. <br>
<br>
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. <br>
<br>
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.<br>
<br>
Regards,<br>
Mark<div><div class="h5"><br>
<br>
<br clear="all"></div></div></div></div></div><br>-- <br><div class="gmail_signature"><div dir="ltr"><font face="georgia, serif">-------------------------------------------------------------------<br>Buddhini Waidyawansa<br></font>Postdoctoral Fellow<br>
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