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

[Larry Weinstein]

Dear Sebastian,

Thank you for your detailed comments.  Here is our new title:
Towards a resolution of the proton form factor problem: new electron and 
positron scattering data

Sebastian Kuhn wrote:
> 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).
deleted 'combined' in most places.
> - 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".
how about: "The electromagnetic form factors describe fundamental 
aspects 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."
OK (mostly)
> - Line 36 p.1: "..., extract it from the ENERGY AND angular dependence..."
changed to 'angular dependence of the elastic cross section at fixed 
momentum transfer'
> - p. 1, top of 2nd column: Sounds a bit awkward and maybe can be simplified: "...that can couple to the two virtual photons."
done
> - line 62 p.2: "...allows US TO use the powerful..."
done
> - Fig. 1: It would be nice to indicate (by thin vertical lines) some typical cuts applied on each of these distributions.
we could do this for the first three, but the width of the last (bottom 
right) varies too much too show typical cuts
> - Bottom of 1st column, p. 3: Quote the relative magnitude (range) of this background.
it is 15% for the worst data point and 5% or less for all the others
> - 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?
yes, the radiative correction cited here is the difference between R and 
R'.  The uncertainty on that last data point is the 0.45% you mention.  
This is shown explicitly in the supplemental information data tables.
> - Line 37, p. 4: "Figure 3 shows the ratio R' VS. EPSILON at... and VERSUS Q2 at...".
They can see that in the figure caption.
> - 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.
we attempted to label the curves rather than to have a legend.  This 
works better in the top plot.
> 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.
ok
> - Fig. 4: The equation in the caption is inconsistent - either it's "G_D" without the square or you have to square the parentheses.
good catch
> - 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.
Replaced with: However, those calculations do not fully resolve the 
discrepancy at higher  $Q^2$

-- 
				Sincerely,
				Larry

-----------------------------------------------------------
Lawrence Weinstein
University Professor
Physics Department
Old Dominion University
Norfolk, VA 23529
757 683 5803
757 683 3038 (fax)
weinstein at odu.edu
http://www.lions.odu.edu/~lweinste/