[Clascomment] OPT-IN: Resolving the proton form factor problem by comparing electron and positron scattering from the proton

[Sebastian Kuhn]

Overall a nice paper - glad to see it come out! I'm not sure, though, whether the title over promises on the results: While there is a nearly 2-sigma deviation of the ratio from being flat and equal to 1, this is not highly significant, and also at a fairly low Q2 where the discrepancy is rather low (as acknowledged in the text itself). So I would argue that this measurement adds important information, but does not - by itself - "Resolve" the TPE issue as promised in the title. Maybe a more appropriate title would be "Towards a resolution" or "New experimental information on..." etc.

The rest are minor comments:
- In line 5-6 of the abstract, the line below Eq. 2, and line 3 p. 5 you repeat the phrase "combined simultaneous" which to me sounds redundant (one of those 2 words suffices). (BTW, the line numbering scheme seems faulty and less than useful; there are both lines and whole pages that are missing numbers).
- The very first sentence is a bit awkward, in particular the word "pieces". Maybe: "Electromagnetic form factors are an essential ingredient in our understanding of nucleon structure". 
- line 18ff p. 1: "Until...understood, the RESULTING uncertainty ... measurements, COMPARISONS TO isovector and isoscalar... QCD [10}, and MEASUREMENTS to extract...form factors FROM PARITY-VIOLATING ASYMMETRIES."
- Line 36 p.1: "..., extract it from the ENERGY AND angular dependence..."
- p. 1, top of 2nd column: Sounds a bit awkward and maybe can be simplified: "...that can couple to the two virtual photons."
- line 62 p.2: "...allows US TO use the powerful..."
- Fig. 1: It would be nice to indicate (by thin vertical lines) some typical cuts applied on each of these distributions.
- Bottom of 1st column, p. 3: Quote the relative magnitude (range) of this background.
- End of 1st paragraph p.4: Is the 0.4-3% for radiative corrections relative to the total cross section ratio? Maybe this could be clarified (i.e., by saying it's 0.4-3% difference between R and R' in Eqs. 1-2.). I assume this is not the "delta_even" contribution? Is it fair to say that you assume an uncertainty of 0.45% on the ratio where this correction is 3%, i.e. at the one data point that is inconsistent with R = 1 in the top half of Fig. 3?
- Line 37, p. 4: "Figure 3 shows the ratio R' VS. EPSILON at... and VERSUS Q2 at...".
- Fig. 3: It would be nice if the legend within the figure would not only match the color but also show the (dashed, dotted or solid) lines associated with the 3 theory curves. Also, the last sentence of the caption should read "The open green circles show the previous world [no "s"] data (AT Q2 > 1 GeV2 for the top plot)." Note the added () - obviously the statement is wrong for the bottom plot.
- Fig. 4: The equation in the caption is inconsistent - either it's "G_D" without the square or you have to square the parentheses.
- line 12 p.5: "calculations are not quite large enough" is clumsy. What you mean to say is that they don't predict a large enough correction to completely resolve the discrepancy at higher Q2.