[Clascomment] OPT-IN: Subthreshold photoproduction of phi mesons from deuterium

[Reinhard Schumacher]

Hello Haiyan et al.,

	I see that the paper has improved somewhat since the first draft that was made available for Collaboration review.  In my view, the paper still lacks a sharp focus.  If you are thinking of sending this to PRL, I think the message has to be much more well defined to have a realistic chance of approval.  As it stands now, the message is just not clear enough.  On the one hand you say in your comments (below), that you want to “present what we have learned at the present time on how to design an experiment to search for the phi-N bound state.”  I don’t believe that goal rises to the level of a PRL paper.  On the other hand, you have some results that are mildly coupled to the experimental goal of detecting a phi-N bound state, but the paper does not make a very strong case for the validity of those results, since it uses up a lot of space on the motivation discussion before it gets to presenting the experimental work.  So I can only repeat my previous recommendation and make th
 e paper less strident at the beginning, and support the experimental results more toward the middle and end of the paper.   Take a look at your last paragraph “We have extracted…”.  It does not “close the loop” with what you wrote on the first page “As mediators of the strong force, gluons play an important role…”.  

	Let us again consider the word “subthreshold”.  With an ideal detector, you could set the photon energy upper limit at 1.57 GeV, and then any phi events you detect are truly from the subthreshold region, since no phis can come from the free proton in that case at any angle.  Because of the vanishing CLAS acceptance in the forward direction, you have no choice but to raise your range on the photon energy from 1.65 GeV to 1.75 GeV to have at least some sideways momentum available to get into the CLAS acceptance.  Figure 2 shows that the free proton events are already leaking in a little bit.  The Fermi motion in the deuteron now is giving enough extra kick to get more events into the acceptance, so in a limited sense you can say these events are from the subthreshold region.  The reaction threshold, per se, remains at 1.57 GeV, and this is the sense that I think most people construe the word.    Your measurement is made “near threshold” or possibly “at threshold”, but not “sub
 -threshold”.  I believe truth in advertising would lead you to delete the prefix “Sub-“ from the word “threshold” in the title, and to make it explicitly clear what you are measuring right in the abstract.

	I don’t understand your response to my question about the E_gammaboost cut.  What do you mean that you want “no events from hydrogen”?  I thought you were using a deuteron target, so there should be no hydrogen around, and anyway your Figure 2 supposedly shows that there are negligible “free” events in your sample.  So it seems your procedure is not really about free hydrogen at all.  In the quasi-free case you may want to limit the effective photon energy interacting with the proton to be less than 1.75 GeV.  I think this muddies the water, but OK, you can do that.  You want to limit the effective photon energy that impinges upon the protons.  Surely that means you want the boosted photon energy to be LESS than 1.75 GeV, not greater than.   I don’t understand the sign of your selection cut.  You say in the paper “We define the subthreshold production by requiring the value of E_gammaboost to be *larger* than 1.75 GeV.”  Can you explain this any better?

	Another item I want to bring to your attention is that you mentioned in your reply to my first comments that “…no QCD molecules in the form of meson-baryon have been experimentally observed…”  This is just not the case.  Have a look at the last ten years of work by E. Oset and collaborators working in the framework of unitary chiral perturbation theory.  By coupling the pseudo-scalar mesons to the octet of ground-state baryons, they are able to find good evidence that a number of N* and Y* states are well-described as meson-baryon states.   Their framework even offers an explanation for the missing baryon problem that I think is quite plausible.


	By the way, you seem to think erroneously that I am involved in searching for an attractive  phi-N interaction using PWA on the proton data from CLAS.   This is not the case:  one of the CMU students is looking at that, but it is not my present area of work.  

That’s all for this round.  I hope the Ad Hoc committee takes note of this discussion and can offer some additional guidance.

Best Regards,
Reinhard.


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Hello All, I have read the draft paper by X. Qian et al on ¨Subthreshold photoproduction
of phi mesons from deuterium¨ . I am surprised that this draft has risen to the level
of Collaboration review. In my view it is scarcely ready for publication. Let me outline
my issues with the draft paper.

Thanks for your careful reading and comments. In the following, we will try to address
the issues that you raised about the draft paper.



The actual scientific result of this paper is very modest (Fig. 4), but the introductory
discussion is strongly worded, promising broad relevance of the following discussion (page
1), but also almost totally irrelevant to appreciation of the results. I would suggest
rewriting the first introductory 3 paragraphs to focus more on what you actually did in
this measurement, and leave out (or greatly reduce) the gloss about the most grand issues
in QCD. This paper has nothing to add to the grand issues, and I strongly recommend
not promising more than you can deliver.

We do not fully agree with this comment. We briefly listed out logic here to clarify
this issue:
• The first paragraph is trying to illustrate the direct role of gluons in the
confinement region. In our point of view, there are three important directions.
The first one is the role of gluons in the nucleon structure (spin, momentum).
The second one is to excite the gluonic degree of freedom in mesons (for example,
the Hall D Program). The third one is the “QCD van der Walls” force (interhadronic
interaction and QCD molecule). We mention the HERA results because
currently no QCD molecules in the form of meson-baryon have been experimentally
observed, though there are indications for the existence of meson-meson molecules.
But theese QCD molecules are natural predictions of multi-gluon exchange, which
is an important piece of the strong interaction. As you can see here, these three
aspects are all closely related.
• The second paragraph is trying to explain why _-N bound state is possible
based on the “QCD van der Walls” picture.
• The third, fourth, and fifth paragraphs are trying to explain how to
search for such a state experimentally. The essential information is we have
learned from simulations and from data (the results of this analysis) in order to
carry out such an experimental search. Our original intention was to present both
results (simulation/data) on similar footings. We already significantly reduced the
discussions in the simulation part as suggested by the Ad hoc review committee.
• The rest of paper focused on how to extract the experimental results on
subthreshold _-meson photoproduction from deuterium.
As you pointed out, our results are limited by the statistics, which we agree. This
is exactly the reason that we need to emphasize/build the physics case here. Otherwise,
why do we care about measuring this differential cross section to 20-30%? The reasons
we care are:
• It is not because we want to search for _-N bound state in this channel (on deuteron
or on nucleon). We understand that the CMU group is currently trying to search
for the _-N bound state with _-meson photoproduction on hydrogen. Dey, Biplab
has contacted us about this issue. We are glad that you are also interested in
this physics. We appreciate your idea in searching for it with the Partial Wave
Analysis. However, in our approach of the direct search, the choice of the subthreshold
production is to produce low-momentum _ mesons in the nuclear medium
to enhance the probability for the production of the _-N bound state.
• It is important to measure the subthreshold cross section in order to design in the
future a dedicated experiment to search for the _-N bound state. A 20-30% cross
section measurement is quite sufficient for this purpose. Such cross sections are
expected to be small.
• For the direct search, the direct K+K− production is also an important background
channel. Our results also shed light on this issue. (see Fig. 2)

For example, the reader finds in the very first sentence ¨As mediators of the strong force,
gluons play an important role in the structure of the nucleon.¨Your result has nothing new
to say about either the structure of the nucleon or gluons. Thus, the reader is misled
from the beginning about the significance of what you are about to present. As another
example, at the end of the first paragraph you gratuitously insert a sentence about the
future existence of an experiment to search for hybrids. This is totally irrelevant to your
message in this paper and only serves as a distraction.

Please see our explanations for the previous point. Our introduction part is perhaps
somewhat unusual, but we believe that it is necessary to motivate the physics and to
make the connection to the future search of _-N bound states.

Another example of the distracting introductory material that I think you can remove
is Figure 1. The simulation you show the reader is for copper, not deuterium, hence not
really relevant to this paper. It shows that the sum of scaler momenta can in some cases
distinguish production channels. However, this is not the method you are using in this
analysis, so again it is irrelevant to the message of this paper.
To first order, the paper could begin with paragraph 4 on page 2.
Also see our earlier explanations. Our goal of this paper is to present what we have
learned at the present time on how to design an experiment to search for the _-N bound
state. We believe the simulation results are an essential piece. As we explained before,
we have reduced significantly already the simulation part of the discussion in the current
version of the paper.

At the bottom of page 2 you point out that the threshold for phi production is at
1.57 GeV, but soon the reader finds that you select the photon energy range of 1.65 to
1.75 GeV for the results you report. Thus, your results are not ¨sub-threshold¨ , but ¨near
threshold¨ . Am I missing something here? You owe it to the reader to acknowledge this,
and to comment on its meaning for interpreting your results.

Thanks to John’s explanation in the previous response to your point. We hope things
are clear now. Here we just want to add one point. Our idea is to use the Fermi motion
of nucleons inside nucleus to produce low-velocity _ mesons. This is our central point of
“subthreshold”, since there is no fermi motion for hydrogen. This is illustrated in the page
2 left column last paragraph. In addition, we are not trying to interpret our results along
the line of searching for the signal of _-N bound state in this channel with deuterium.

The discussion around Equation (2) could be made more convincing. The reader is
not given a picture (literally) of what the proton cross sections look like, or how good the
fits are. By introducing the equation and the ”ai” parameters you are saying they are
somehow important to the results. But you don´t show them in either a graphical or a
tabular form, so there is little value in introducing this to the reader. I think you should
either complete this discussion or leave it out altogether. I assume the technical review
checked that this all makes sense, but your draft paper raises a pretty big ref flag in the
mind of the reader at this point.

This is an excellent point. We are well aware of this point. The major issue we face
here is that the g11 cross section results are not released yet. Fig. 16 in our analysis note
showed two examples of our fitting. Using this event generator, we extracted the cross
sections at quasi-free kinematics in the entire g10 photon energy range. Fig. 21 shows the
comparison of the results between the two data sets. If the referee(s) asks this question,
we plan to send them the example plots together with the extracted cross sections at
quasi-free kinematics to support this point.

The discussion of the ”special cut” on the quantity Eboost starting at the bottom
of page 3 column 1 is pretty opaque. You say you *select* the near-threshold region by
requiring this boosted photon energy to be *greater than* 1.75 GeV. I would have thought
it should be LESS than this amount. Is there some way you can make all this more clear
to the reader?

The “subthreshold” events require a) no events from hydrogen, b) signals for nuclear
targets due to fermi motion. In the hydrogen case, Eboost _ Egamma. Since we are looking
at the data between 1.65 and 1.75 GeV. the subthreshold events are required to have a cut
Eboost > 1.75GeV , which by definition exclude any events from hydrogen. From Fig. 2,
our g10 data are dominated by the “subthreshold” events. However, when we calculated
the cross section, we need to exclude the contributions from the “abovethreshold” events.

In the later part of the same paragraph, I am afraid I don´t understand the origin of the
¨cusp¨and how it is related to Eboost. In fact, the theoretical calculation is not described in
any detail at all. The paper would be better if you did this. This is crucial to your message
because your claim is that these results provide a ¨benchmark¨for future experiments on
heavier nuclei. Indeed, how well the results compare with simple theory is the measure
of whether you can say that the results on deuterium show that our understanding of a
simple process is under control. At a minimum, give the reader a literature reference to
the ¨simple theoretical calculation¨you are adapting. Alternatively, as the author of the
model (Jean-Marc?) to provide a paragraph of discussion for this paper.

In response to Dan’s comment, we removed the discussion of the “cusp” stuff. The
origin of this structure is due to the finite bin size and the Eboost cut. More information
can be found in our response to first round comments of the ad hoc review committee.
We do describe the simple theoretical calculation. The calculation is done together
with the event generator. It is not from the Laget’s calculation. In page 3, right column,
we mentioned “In addition, our data have been compared to a simple theoretical calculation
based on a quasifree picture with the _-meson photoproduction differential cross
section from the proton based on the g11 data [?].”. We added another sentence in the
manuscript “The principle of the simple calculation is the same as the event generator
used in MC.” to emphasize this point. We are not trying to search for the _-N bound
state in this particular channel. Second, our results demonstrate that a simple quasi-free
picture together with a reasonable model of _-meson photoproduction cross section on
hydrogen can reasonably estimate the subthreshold production rates. Other effects like,
the FSI, need to be taken into account in estimating the production rates from nuclei.
>From this point of view, the exact production mechanism of _-meson photoproduction on
the nucleon is not that crucial. Nevertheless, we do appreciate your idea of searching for
the signature of _-N bound state from the _ production mechanism on the nucleon.

One final complaint: you have 11 lead authors listed, some of whom I don´t recall giving
talks on this subject at Collaboration meeting on this or any other topic. We generally
have just a handful of lead authors on CLAS paper. Were all these people really crucial
to obtaining these results?

For this analysis, we had weekly _ analysis meetings for a long time discussing the
analysis and the results. We worked as a team in this case, every lead author made contributions
here and there at different stages of the analysis. A number of your comments
have good overlap with earlier comments raised by Daniel Carman. We have posted a
response to Daniel’s comments and we have posted a revised version to the review website
dated April 8, 2010. So please take a look at our response and the April 8 version of the
paper. We will update another version soon once we hear from all. Thank you again for
your careful reading of this paper.

Looking forward to the next draft, Reinhard