[Clascomment] OPT-IN: Demonstration of a novel technique to measure two-photon exchange effects in elastic e(+/-)p scattering

[Mikhail Osipenko]

Dear Authors of TPE paper,

I have few suggestions for this draft:

1) abstract, second sentence: "limited experimental evidence" is not universally recognized yet, I would suggest more cautious "there is no unambiguous direct experimental evidence",

2) Eq.1 is not fully correct - A_loop is a sum over all leptons and all quarks, hence I am not really sure what charge has to be put in front of it,

3) sentence right after Eq.2 - probably you wanted to say "thus identical for electron and POSITRON" scattering." And proton antiproton as well - but it is better to omit proton charge discussion as irrelevant to this paper.

4) in Eq.3 use curved brackets to distinguish better from brackets of Re(),

5) below Eq.4 use "divergenceS cancel",

6) Eq.5 - \delta_2\gamma and \delta_br. - are finite parts of TPE and bremsstrahlung, so that R_2\gamma in Eq.6 is also the finite part of TPE contribution. I don't know what sense does it make to separate it from its divergent part.

7) you select elastic reaction by a number of angular cuts, but calculating RC using a naive prescription applicable to a measurement with only scattered lepton detected and a cut on its energy. Be prepared to face questions: why do you apply incompatible RC corrections?

Best Regards,
                  Mikhail.