[Clascomment] OPT-IN: gamma n --> pi- p Differential Cross Section Measurements with CLAS

[Volker Burkert]

Dear lead authors: 
This is a well-written paper with precise results that will have impact on future coupled-channel 
analyses and knowledge of resonance parameters, even in finding possibly new high mass states. 


I have only very few, relatively minor, comments. 

1) Abstract:  "A model-dependent correction". I think this does not belong in the abstract. 
The correction is model-dependent, but it turns out to be quite small and doesn't deserve 
a prominent place in the abstract. I suggest to take the "model-dependent" out. 

2) TABLE I: You list the relative error on the helicity amplitudes to demonstrate the need for 
experiments on the neutron. Of the 9 entries in the table the relative errors for the 
neutrons are significantly larger compared to the protons cases in only 4 entries.  If you replace
the relative errors with the amplitudes and the absolute errors on the amplitudes, there are 6 out of 
the 9 entries where the neutron amplitude has larger uncertainties than the proton. 
I suggest to do just that. It is also better to see what the absolute value of the amplitude is.

3) line 132: "produced AN APPROXIMATE toroidal magnetic field".  

4) line 140: Is it true that the phi resolution in the drift chamber is equal to the theta resolution. 
   The phi resolution must be theta-dependent as at theta=0, phi is undefined, i.e. d(phi)=360deg.  

5) Fig. 3 caption: How were the contributions from the target cell removed? Were empty target 
data taken and subtracted, or were additional tighter cuts used around the target?.  

6) line 224: "Fig. 4(a) shows the missing momentum distribution after this cut." In fact this is not 
shown, what is shown in Fig. 4(a) is the missing momentum distribution and the cut that was applied
to eliminate the majority of the rescattering events".

7) "dead wires", etc.  
I suggest to replace those characterizations with more technical terms, such as "non-reporting sense wires, 
or disconnected sense wires" and "disconnected or high-voltage channels".  

8) line 256: "cut from the analysis" => "eliminated from the analysis"

9) Fig. 19:  The bottom panels should be labelled as M3- (not M3+). I think they show the N(1675)5/2- pole
 as in the top panel, which is labelled correctly.   

10) line 904-906. You may want to also point out that the data don't end at the "fourth resonance region"
 but extend far into the poorly studied high mass region where many resonances are expected to exist, and
these dat will help in coupled channel analyses that will explore this region in the search for these states. 

11) "the use OF deuterium". 

12) line 938: The N(1650)½+ is not ½+ but ½- spin-parity. 


Volker