[Olympus] Fwd: Your_manuscript LY14962 Henderson/thoughts on comparison with VEPP3

[Belostotski, Stanislav]

Dear colleagues,
I have attached the file with some thoughts on our results.
Sorry i cannot join OLYMPUS meeting today.
Best regards stanB

On 02.12.2016 22:02, Douglas K Hasell wrote:
>
>
>                                                        Cheers,
>                                                                Douglas
>
> 26-415 M.I.T.                                  Tel: +1 (617) 258-7199
> 77 Massachusetts Avenue                        Fax: +1 (617) 258-5440
> Cambridge, MA 02139, USA                       E-mail: hasell at mit.edu
> <mailto:hasell at mit.edu>
>
>
>
>
>
>
>
>
>> Begin forwarded message:
>>
>> *From: *<prl at aps.org <mailto:prl at aps.org>>
>> *Subject: **Your_manuscript LY14962 Henderson*
>> *Date: *November 30, 2016 at 12:02:58 EST
>> *To: *<hasell at mit.edu <mailto:hasell at mit.edu>>
>> *Reply-To: *<prl at aps.org <mailto:prl at aps.org>>
>>
>> Re: LY14962
>>    Hard two-photon contribution to elastic lepton-proton scattering
>>    determined by the OLYMPUS experiment
>>    by B. S. Henderson, L. D. Ice, D. Khaneft, et al.
>>
>> Dear Dr. Hasell,
>>
>> The above manuscript has been reviewed by our referees.  We ask you
>> to consider the appended comments from the reports.
>>
>> While we cannot make a definite commitment, the probable course of
>> action if you choose to resubmit is indicated below.
>>
>> ( ) Acceptance, if the editors can judge that all or most of the
>>    criticism has been met.
>>
>> (x) Return to the previous referee(s) for review if available.
>>
>> ( ) Submittal to new referee(s) for review.
>>
>> With any resubmittal, please include a summary of changes made
>> and a brief response to all recommendations and criticisms.
>>
>> Yours sincerely,
>>
>> Kevin Dusling
>> Associate Editor
>> Physical Review Letters
>> Email: prl at aps.org <mailto:prl at aps.org>
>> http://journals.aps.org/prl/
>>
>> IMPORTANT: Editorial "Review Changes"
>> http://journals.aps.org/prl/edannounce/PhysRevLett.111.180001
>>
>>
>> COLOR REMINDER:
>>  You have requested to have some color figures in the print
>>  journal (1,2).  If your paper is accepted you will
>>  be invoiced for the cost before your paper is sent to our
>>  production vendor for the amount of US$1495.
>>  (See: http://journals.aps.org/authors/color-figures-print)
>>
>> ----------------------------------------------------------------------
>> Report of Referee A -- LY14962/Henderson
>> ----------------------------------------------------------------------
>>
>> The manuscript describes a measurement of the hard two photon exchange
>> in proton lepton-scattering, which is presumed to be a contributor to
>> the discrepancies observed between Rosenbluth and polarization type
>> measurements. The field of lepton-nucleon scattering is currently a
>> highly relevant field in modern-day nuclear physics due to the recent
>> measurements of the proton charge radius in muonic hydrogen, which
>> exhibit a strong disagreement between electron and muon results. While
>> the measurements described to not directly address this issue, they
>> are certainly relevant and serve to motivate theoretical efforts
>> related to the nucleon structure.
>>
>> The manuscript gives a concise and accessible description of the
>> OLYMPUS experiments and the measurement, but unfortunately does not
>> give a clear conclusion as to whether two photon effects are driving
>> the mentioned difference, but rather, motivates further research on
>> this issue. I find the manuscript worthy of publication in PRL since
>> it addresses important issues, even without reaching a firm
>> conclusion. I do however have a few comments which should be addressed
>> before publication:
>>
>> 1. The authors claim that their results are consistent with the VEPP
>> and JLab results. However, both these experiments, whose results have
>> been published in PRL claim to support the resolution of the form
>> factors discrepancy with two photon effects. This claim seems to be at
>> odds with the claim made by the current manuscript, who mention that
>> their resort to not serve to resolve the discrepancy. Since the
>> uncertainties in all three experiments are of similar size, I would
>> expect the claim to either be different or for the authors to state
>> more clearly why they claim different than the other two experiments.
>>
>> 2. Related to my previous comment. Since the results from the other
>> two experiment have already been published, they should be presented
>> on the same figure used to present the new results (Fig. 2 in the
>> current manuscript) so that the reader may better gauge the
>> consistency on the results. Other, less precise, world data, could
>> also be included, but it's inclusion is likely of no benefit to the
>> reader.
>>
>> ----------------------------------------------------------------------
>> Report of Referee B -- LY14962/Henderson
>> ----------------------------------------------------------------------
>>
>> See Attachment: ly14962_report_1_b.pdf
>>
>> ----------------------------------------------------------------------
>> Report of Referee C -- LY14962/Henderson
>> ----------------------------------------------------------------------
>>
>> This paper presents important results and should be published in PRL.
>>
>> Olympus is the third of a series of recent high-precision experiments
>> focusing on two-photon exchange (TPE) contribution to lepton-proton
>> scattering, via the measurement of the ratio of (e^+p) to (e^-p)
>> elastic cross sections.
>>
>> The paper is well written. The ratio R_2gamma and its role in
>> understanding the discrepancy between polarized and unpolarized proton
>> form factor measurements are clearly explained, as well as the
>> practical procedure to extract R_2gamma from the experiment.
>>
>> A very complete set of results is presented over a large range of
>> epsilon (or Q2, being here correlated to epsilon by the fixed beam
>> energy). The obtained values of |R_2gamma-1| hardly exceed 1%,
>> suggesting that TPE is not enough to fully explain the discrepancy in
>> proton form factor measurements, at least for [Q2 < 2 GeV2] which is
>> the presently explored range for R_2gamma. This naturally calls for
>> future high-precision (e^+p)/(e^-p) experiments at higher Q2, where
>> large values of R_2gamma would be necessary to explain the form factor
>> discrepancy.
>>
>> I have three main remarks, listed below, and a few minor comments.
>>
>> 1) The whole experiment is about radiative corrections (RCs) and how
>> they are handled. As said in the text, higher-order RCs (i.e. beyond
>> order alpha^3) are becoming significant, in view of the sub-percent
>> precision reached experimentally. The authors could say more clearly
>> to what extent higher-order RCs become (or will become in the future)
>> a limitation for interpreting the TPE effect.
>>
>> Exponentiation is only an approximate method to estimate the
>> higher-order RCs. I see the difference between exponentiated and
>> non-exponentiated results (difference |(a)-(b)| in table II) more like
>> a theoretical uncertainty attached to the order alpha^3 calculation,
>> that should be taken into account when comparing data with theoretical
>> curves (fig.2). Figure 1 is also suggestive of how the experimental
>> determination of R_2gamma is RC-scheme dependent. High-precision
>> experiments like this one really call for improvements in the
>> calculation of RCs (higher orders in alpha, but also reducing the
>> approximations made at order alpha^3) and this could be said in the
>> paper.
>>
>> 2) I have several comments on one sentence about the multi-interaction
>> events: "This method compared the relative rates for lepton-lepton
>> coincidences in the SYMB with the rates for simultaneously detecting
>> an additional ~ 2 GeV lepton from lepton-proton elastic scattering
>> [17,30]."
>>
>> - The word "relative" is ambiguously placed: what is relative to what?
>> is it:
>>
>> "the rates for lepton-lepton coincidences in the SYMB" relative to
>>
>> "the rates for simultaneously detecting an additional ..."
>>
>> or something else? (it could be relative rates compared to other
>> relative rates).
>>
>> - The word "simultaneous" is ambiguous. you probably mean an
>> additional lepton in accidental coincidence with the true
>> lepton-lepton coinc.?
>>
>> - This additional lepton: you should add in which detector (the
>> SYMB?).
>>
>> - Why is this MIE method finally chosen? is it because it gives the
>> smallest uncertainty (the 0.36% of next sentence)? it's not clearly
>> said. if yes, can one get a clue as to why it gives the smallest
>> uncertainty?
>>
>> 3) In the conclusion paragraph, some sentences could be made clearer
>> by adding the words marked below between brackets:
>>
>> "TPE calculations that bring the form factor ratio measurements into
>> agreement at large Q2 predict a larger effect [in R_2gamma] at the
>> energies directly measured so far".
>>
>> "Therefore, it is not evident, nor ruled out, that TPE is driving the
>> difference [in form factor ratio measurements] at large Q2".
>>
>> A few minor comments:
>>
>> - The background in the elastic samples: is its origin
>> known/understood?
>>
>> - The background subtraction procedure: on what is it based?
>>
>> - The similarity of background rates for e+ and e- modes: is it
>> expected?
>>
>> - The (possible) systematic error coming from having run mostly with
>> only one toroid polarity: is it addressed somewhere?
>>
>> - Table 2: column 1 ("bins") is not very useful.
>>
>> - Fig. 2, curve labeled "Blunden N": shouldn't it be referenced as [4]
>> (instead of ref.[31] in the text, which should apply only to the curve
>> "Blunden N+Delta")?.
>>
>> - Ref. [27]: what is the document type (MIT internal report?)
> =3D
>
>
>
>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Oly_referee_comment.pdf
Type: application/pdf
Size: 218599 bytes
Desc: not available
Url : http://mailman.mit.edu/pipermail/olympus/attachments/20161205/031114a=
4/attachment.pdf