[Clascomment] OPT-IN: Photoproduction of pi0 on Hydrogen using e+e-(gamma) detection mode with CLAS

[Volker Burkert]

Dear lead authors: 
This is an interesting paper and I hope it will make it into PRL.
Here are a few comments:

Abstract:  
 The abstract should convey the most important results in a compact form. The sentence 
“Our data appear to favor the Regge pole model ….” is not a result. Either drop the 
‘appear to’ or the sentence should be removed from the abstract. 
Introduction: Since the paper is targeted for PRL it would be good to make the intro a 
bit more appealing to a broader audience. I don’t have any specific suggestions, but 
you may want to formulate something around the unusual properties of the pion. 
 (Note: the broad audience aspect is important for the acceptance of papers in PRL. 
The referees are specifically asked to look at this aspect.) 
It is a nearly massless particle, although as a qq-bar meson with constituent quark masses 
of 350MeV one would expect a much larger rest mass. It has a role in chiral symmetry breaking, 
and may play an important role in quark confinement and the stability of the proton.  
All of this makes it interesting to understand its properties when probed in dynamical 
processes such as is discussed in this paper ………..   

Line 44:  “The oldest model..” replace with something like “The earliest model..” or “The first model…” . 

Line 57:  “(A_2,etc.)   ..  (rho,etc)”  => delete the ‘etc.’  or give concrete mesons by name. 

Line 61-63: The sentence “ While the dip at t=-0.5 GeV^2 is present in pi^0 data , it is not 
in the recent beam asymmetry data on eta photoproduction [5]”.   Reference [5] refers to the 
GlueX polarized beam asymmetry data, where the dip at t=-0.5 is not present. However it is 
not present in both, the pi0 and the eta data.  

Line 69:  “ However, to “explain” …… they remove…, ad hoc” replace with “However, to 
“reproduce” ….they remove …”  (it “explains” nothing).

Line 81: “..while the other models are “good” for more ..” => I suggest to replace “good” 
with “applicable”. 

Line 95-97: The excursion to DVCS (which is electroproduction at small t and is unrelated
 to the topic of this paper) seems to only serve the purpose to include reference [12], and 
not even the published version of that paper but a conference proceeding). I think it is a 
distraction and may just be confusing to the reader. 

Line 151:  “..using a tagged photon beam ...”  => “ .. using an energy-tagged photon beam,..”. 

Fig.1:  Add the color code normalization as side bars. Also, instead of using M_E(p,e+,e-) as 
a missing energy I suggest to use the notation E_X(p,e+,e-) as missing energy (E) and define 
it properly in the text, in analogy to the missing mass  (M) notation M_X(,,,).  The “X” denotes 
the “missing part”. 

Line 305 – 317: You may want to add acknowledgment to the German Funding Agency, if 
they provided funds. 

References: 
[16] and [19]. As in references to Hall A [15] and to GlueX [5] you may want to add 
“CLAS Collaboration to the reference [16] and [19]. 


Volker Burkert