[Clascomment] OPT-IN:Measurement of the free neutron structure function using spectator tagging in inelastic d(e, e' p_s)X scattering with CLAS

[Volker Burkert]

Dear Slava and lead authors, 
Congratulations to the paper, which obviously required an enormous amount 
of work!  

I have only a few comment that you may want to consider.
 
1) In the introduction on line 57 you mention the sigma(n)/Sigma(p) ratio for 
the S11(1535) resonance. Unfortunately, inclusive unpolarized data can't really 
say much about this as in this mass range there are two resonances dominant 
resonances, in addition to the S11 the D13(1520) is also present and has in 
fact a large electromagnetic coupling. So, unless one can disentangle the two 
resonances, which requires exclusive measurements and a partial wave analysis, 
one can really not say much about the above ratio. I would hope that the exclusive 
data e+n-> e+ proton +pi- from the BONuS experiment will have something 
to say about resonance production on neutrons. My suggestion is to either drop 
the S11 or say something related to exclusive data. 

2) Line 764, the bullet discusses the Cherenkov counter threshold. It is not clear 
to me why  one would lower the threshold for particle with momenta above 3 GeV 
to reduce contributions of negative pions.  It would seem it is exactly the opposite. 
Raising the threshold above 3 GeV/c particle momentum would reduce the pion 
efficiency as the signal from pions grows slowly only above threshold, thus reducing 
pion contamination while keeping electron detection efficiency high.  

3) Line 788: No reader will understand what a chi^2 = 4 on the fitted particle trajectories 
in the drift chambers means, unless you show a distribution or give an average chi^2
for comparison. 

4) Figure 10: Vertical axis is labeled F_{2n} while all other graphs have F_2^n.  Also, 
I don't understand the systematic error band. The band shows the systematics for 
one of the analysis. However, since both data sets are cross normalized to each other 
and to the CJ fit what is the meaning of this band. What might make sense is to show 
the difference in the systematic uncertainties for the two analysis. My guess is that 
they would be much smaller than what is shown. My suggestion is to eliminate the band 
and say that only statistical uncertainties are shown. 

5) Figure 20: This is the main result, so it should give a clear message. Unfortunately, 
this is not the case. The message is confused by putting three data sets on top of 
each other. Two of the data sets clearly show that the cuts at W*>1.4 and 1.6 are 
to low to eliminate resonances. W*>1.8 might just be sufficient to be called "DIS".  
My suggestion is to either use only W*>1.8 GeV as lower limit or even use W*>2GeV. 
The latter would be a more generally accepted lower mass cut for the DIS regime. 
The summary clearly states that the energy used in the experiment was too low to 
say much about the high x region and a new experiment is forthcoming.

6) There are a number of incomplete references, e.g. [30], [55], [60], [69].  

Volker Burkert