OPT-IN: Measurement of two-photon exchange effect by comparing elastic e± p cross sections

[Daniel Carman]

Dipak et al.,

I have read through the draft TPE paper dated January 29. For the most part I think the
paper is well constructed and clearly written. I include my comments (physics, style,
and grammar) below. Let me know if you have any questions.


					        Regards,
						
						Daniel

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General:
 - Use units of c=1 throughout as you are not consistent in your notation.
 - You switch back and forth between "beam line" and "beamline". Be consistent.

Page 1:
 - Line 15. Use "... TPE effects has been observed."

Page 2:
 - Line 70. Use "... magnetic moment ...".
 - Line 97. Use "... of the intermediate ...".

Page 4:
 - Fig. 3. The representation of CLAS (the gray shadow) is far too dim and should
   be made darker.
 - Line 305. Use "... torus magnet ...".

Page 5:
 - Line 310. Use "... the spatial positions ...".
 - Line 317. Use "... downstream of CLAS.".

Page 6:
 - Line 368. Use "... between the Moller and ...".

Page 7:
 - Line 391. Use "Note that the distributions ...".
 - Line 393. Superscript problem on electron.
 - Line 407. Do not capitalize "Corrections".
 - Line 438. Use "... momentum corrections, ...".
 - Line 448. Use "... with tracks of both positive ...".

Page 8:
 - Line 484. Use "... as a $p$ candidate.".
 - Line 486. Use "... identified as an elastic ...".
 - Line 571. Use "... for all but the ...".

Page 10:
 - Line 605. Use "... of sector 3 were removed ...".

Page 11:
 - Line 670. Use "... energy distributions of the ...".

Page 12:
 - Line 734. "OPE" has not been defined in the paper.
 - Line 734. Use "... between the OPE and TPE ...".
 - Comment on radiative corrections. Because of your tight kinematic cuts it seems that you
   are also minimizing the radiative correction effects. This might be worth pointing out.

Page 13:
 - Line 768. Use "Refs. [74,75].".
 - Line 792. Your assigned scale uncertainty of 0.3% (15% of the correction) seems completely
   arbitrary. Can you justify why this uncertainty for an unknown correlation (as you point out)
   is reasonable?
 - Line 844. Use "Charge independence of track reconstruction:"

Page 14:
 - Eq.(17) should end with a period for proper punctuation.
 - Line 887. Spurious "cut" included.
 - Line 888. Need units on z-vertex limits here.

Page 15:
 - Line 902. Use "Acceptance correction:".
 - Line 920. Add a period at the end of the sentence.
 - My sense of reading your conclusions of the value of the measured TPE effect from this
   analysis as shown in Fig. 19 is that you are fully biased by what you expect the effect 
   to look like (i.e. by what is needed from TPE effects to explain the form factor discrepancy). 
   I think that this inherent bias affects how you interpret and discuss your results. Regarding
   Fig. 19 you state (line 939) that the data at Q2=1.45GeV2 shows a moderate epsilon dependence. 
   The data in my opinion do not support this statement by any statistical measure. One data point 
   from your measurement (eps=0.4) is statistically away from unity (and by less than 1 sigma). The 
   other 4 are fully consistent with unity to 3 significant figures.
 - My disagreement with your discussion of the results continues with Fig. 20. Here you state 
   (line 964) that the data are consistent with little or no Q2 dependence. Certainly this does not 
   meet the "eye test" when I look at the data at eps=0.45. I would agree with this qualitative 
   statement only for your eps=0.88 data.
 - Fig. 19 caption.
   - Line 1. Include units after 0.85.
   - Line 4. Use "The filled black squares show ...".
   - Next to last line. Use "... figure is a linear fit ...".

Page 16:
 - Line 992. Use "Ref. [16]".
 - Line 1008. Use "... we do not include these ...".

Page 17.
 - Fig. 20 caption.
   - Line 3. Use "... is our CLAS 2013 result ...". Also include the reference here for the
     CLAS 2013 result.
 - Line 1023. Use "Refs. [21,31]".
 - Table IV caption. Last line. Use "... $\chi^2$ value ...".
 - Line 1045. Remove comma after "factor".

Page 18.
 - Ref. [53]. Use "AIP Conf. Proc.".