[Clascomment] OPT-IN: Measurements of ep->e'pi+n at 1.6 < W < 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS

[Sebastian Kuhn]

I have one general comment and a few minor ones. 

The general comment: I am a bit concerned about the strong distinction made (throughout the article) between "quark contributions" and "non-quark contributions" to the transition amplitudes. The reason this concerns me is that, in general, the only correct fundamental theory of the strong interaction - QCD - would stipulate that EVERYTHING is due to quarks and perhaps gluons - but that's not meant here. The explanation given in Fig. 1 makes it a bit clearer, but I'm afraid that the three diagrams shown are not really orthogonal to each other - they just come from different models (one being a constituent quark model, the other using mesons and baryons as effective degrees of freedom). At the least, it should be spelled out that this distinction is made within a specific model (and that model should be explained a little more) - the explanation in lines 62ff is less than clear in that respect. The half sentence in the 3rd line from the bottom of the abstract ("...indicating si
 gnificant...even at large Q2") becomes largely unintelligible without this context (at least to someone who isn't fully immersed in the CQM and its various extensions). Even accepting this framework, I am not entirely convinced by the discussion in Section VII.A. (BTW, why have a section "A" when there is no section "B"?): While it is true that the SQTM seems to under-predict the (1675) 5/2- A1/2 and A3/2 amplitudes by a large factor, I would say that the 1680 5/2+ amplitudes are fairly well reproduced by the dashed-dot curves, at least qualitatively. Again, maybe there is a significant under-prediction of the A_3/2 strength at large Q2, but can we really make a strong statement about the contribution from the meson cloud based on these data, especially taking statistical, systematic, amplitude extraction and model uncertainties into account? My own tendency would be simply to state that the CQM and in particular the SQTM seem not to be able to fully explain the results, ind
 icating that more complete models are needed.

Now the minor comments:
L25: "wide space-time range"? Pls clarify
L76-68: remove both "the" in front of models
L251: "minimum energy" - maybe better "significant less energy"
L283: Replace "<" with "=" or "\approx"
L301-2: The axes are reversed - it's "particle beta vs. momentum".
L343: "invariant mass of the elastic peak" is a bit unclear - are you talking about W in INCLUSIVE p(e,e') or what?
Fig. 7: These plots look a lot "cleaner" (less background) than the sample in Fig. 6. This should be explained (maybe Fig. 6 is not even needed?)
L496ff: I am a bit confused about how you actually accounted for external radiation. I assume (but it isn't clear) that "THR_RAD" in Eq. 9 refers to a generator that produces events distributed according to the cross section INCLUDING INTERNAL radiative effects, but no external radiation? Still, GSIM does not take care of ALL external radiative effects - there is of course INITIAL radiation of the electron entering the target which usually is NOT part of GSIM. Maybe this could be explained more clearly?
Figs. 19 and 15: These figures appear "out of sequence", i.e. they are referenced in the text either before (Fig. 19, L560) or after (Fig. 15, L691) other figures that appear before/after them. Also, Fig. 19 is very squished - no statistical nor systematic uncertainties are visible and it is very hard to judge for the reader whether indeed there is "global agreement" (L560).
Figs.13 and 14: Some of the solid lines are changing colors back and forth (red to black) without any apparent reason or explanation.
L576-580: This statement appears quite a few times throughout the paper and may be a bit redundant here.
L587: You also present examples of the theta-* dependence of reduced cross section. In fact, I would like to see more of those than the many similar plots on the phi* dependence (what do I learn from Fig. 18 that Fig. 17 doesn't tell me? L745-748 sounds pretty weak...)
L692: "...consist of THE present..."
L700: "...approaches of UIM and DR HAVE BEEN described..."
L727: no "THE"
L728: no "," (comma) 
End of Section VIII: You reflect on future data that will help better determine of transition densities etc.; in this context, you could also mention the wealth of polarization data in the same kinematic regime (A_LU, A_UL, A_LL) that have been taken and are presently being prepared for publication (mostly by P. Bosted). 

- Sebastian