[Clascomment] OPT-IN: Near Threshold Neutral Pion Electroproduction at High Momentum Transfers and Generalized Form Factors

[Volker Burkert]

Lead authors:
I have read this paper with great interest. It is generally well written, but I have a few comments that are meant to improve the paper. 

1) Abstract. 
 It is stated that "Our result suggests that the quark-gluon description of nuclei is viable even at energies lower than 1 GeV".   

I think this statement goes beyond what our data support. The only observation is approximate "dimensional scaling".  Dimensional scaling knows nothing about quark-gluon dynamics, it is a prediction based on point-like interactions between a number of constituents. If the statement had been that the "data are approximately consistent with dimensional scaling predictions" that would be fine.

2) line 17/18: The sentence "..the fact that scaling laws, being derived from pQCD, ... are.." should be changed. 
Scaling laws are more general than pQCD and don't need the specific constituents of pQCD, i.e. quarks and gluons. Just point-like interaction between elementary fields is needed. 

3) line 71: "resonance mechanisms are suppressed shown by low energy studies [25]".  

In that reference it says "The 3He(γ, p)d and 4He(γ, p)t differential cross sections at 90° in the c.m.s. have been measured in the γ-ray energy region 200–500 MeV using a liquid helium target and spark chamber technique. The 4He photodisintegration shows a resonant behavior in the energy interval 240–330 MeV, not observed in the 3He process."

In contrast to that statement our data seem to show a significant resonant behavior below 500MeV, likely coming from the high energy tail end of the Delta(1232) resonance. Also our data in Fig. 3 show a structure near 1 GeV which could be due to resonance contributions to the reaction.   

4) line 205-210.  "We estimate that the uncertainty of the yield extraction due to remaining background events is (2.30 +/-0.63)%"  

What does that mean?  An error (0.63%) on an uncertainty (2.30%)? Or is the yield 2.3 % and its error 0.63% , or does the background subtraction vary from 1.67 (=2.3-0.63) to 2.93 (2.3+0.63) ? Please clarify. 
Same question on the second part of the sentence (0.331 +/- 0.035)%.  Here I want to also ask, why do we bother with uncertainties of 0.035% on a number of 0.331% ? Also, why so many digits for such small numbers?  

Next sentence, why so many digits (1.0338 +/- 0.0017) for a correction factor that is close to 1, and what does the uncertainty +/-0.0017 on that value mean?

5) line 227: 11.39% => 11.4%.  

6) Fig. 3: A discussion is needed on the deviations from the scaling curve near 1-1.2 GeV. Why do we still believe the scaling interpretation is valid if we deviate by up to 4 sigma for one point. Could it be due to distortions from resonant behavior? THere are several resonances that may contribute. It would be nice to have a calculation from a reaction model. Any chance of getting some calculation from Jean Marc Laget that shows resonance suppression? 

7) Line 323 & 327: the reaction is initiated by a "photon beam", not a "lepton beam". 

8) References [32], [33], [34]. Incomplete references. If available give full link for web access.