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

Sun Nov 26 11:14:13 EST 2017

Dear Michael et al,

I have read through your draft of the g12 pi0 production analysis and 
include my comments below. The paper certainly needs work to improve
the grammar and the style. Hopefully I have managed to capture everything
below. If you have any questions, let me know.

Regards,
Daniel
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General:
 - Spell out acronyms the first time that you use them. These include
   QCD, pQCD, DVCS, CM, etc.
 - You are not consistent with your energy notation. Sometimes you use
   "E" for the photon energy and sometimes you use "E_\gamma". I prefer
   the later for clarity.
 - If this paper is to be accepted into PRL, it must be written with the
   idea to make it broadly interesting to both experts and non-experts.
   There are several notable areas where the discussion is too esoteric
   and specialized. I point out some examples below.
 - Be consistent with usage of "handbag" vs. "Handbag".
 - As written, there is no physics conclusion from this paper. Your message
   is that you have performed a measurement using a different final state
   and that your data compare better with one model approach over another.
   This is not satisfying for the reader.
 - The figure captions should not be "centered", but left justified.
 - Sometimes you write $\gamma p \to p \pi^0$ and sometimes $\gamma p \to \pi^0 p$.
   Be consistent throughout.

Page 1:
 - Abstract:
   - Line 2. Use "... decay and the $e^+e^-$ pair ...".
   - Line 3. Use "... CLAS detector at the Thomas ...".
   - Line 6. Use "The final state particles $p$, $e^+$, and $e^-$ were detected
      in CLAS, whereas the ...".
   - Line 7. Use "... sample has quadrupled the ...".
   - Line 8. Use "... $E = 2$~GeV, and appear to favor ...".
   - Final sentence has no physics content. What does this imply and why is
     this statement important?
 - Line 13. Use "... models, and QCD.".
 - Line 17. Use "... production regime and the ...".
 - Line 27. Use "... [3], as well as ...".
 - Line 35. Use "... $t$-channel $J^{PC}$ quantum ...".
 - Line 46. Use "... $t$-channel ... numbers, along with ...".
 - Line 56. Use "... zeroes in the Regge residues.".
 - Line 60. What is a "nonsense wrong signature zero"? This is definitely jargon.
   It needs a reference and a sentence of explanation/context.
 - Line 62. Use "... in the $\pi^0$ data.".
 - Line 69. Use "However, to explain the lack of a dip at $t \approx -0.5$~GeV$^2$ in
   $\eta$ photoproduction, they removed the standard wrong signature zero in an ad hoc
   manner." What is a "standard wrong signature zero"? This is jargon.
 - Line 72. Use "... $t$-channel ... \omega$, and $b_1^0$ exchange, but not ...".
 - Line 74. What do you mean with the notation "$\rho \times$ Pomeron cuts" and
   "$\rho \times f_2$ cuts"?

Page 2:
 - Line 77. Use "$u$-channel".
 - Line 79. Use "$t$-channel".
 - Line 88. Use "complementary".
 - Line 95. Use "... measurement of the beam spin asymmetry ...". Actually I am not sure
    why you are bringing up this measurement here. It does not seem relevant.
 - Line 100. Use "... transverse momentum, which corresponds to ...".
 - Line 101. What is this angle theta? It needs to be defined.
 - Line 102. Use "... may extend to the ... case proposed in [10]."
 - Line 106. Use "... the production cross section.".
 - Line 107. Use "Binary reactions in QCD with large ...".
 - Line 108. Use "... between the colliding ...".
 - Line 110. Use "... [3] provide a simple recipe ...".
 - Line 114. Use "... of these counting rules ...".
 - Line 115. Use "As was first observed at SLAC by Anderson {\it et al.} [13], the ...".
 - Line 118. Do not put [13] here but earlier as I suggest.
 - Line 119. Use "$s = 6$~GeV$^2$ ...".
 - Line 121. Use "$s = 10$~GeV$^2$.".
 - Line 124. I do not understand your notation of $d \sigma/dt(\vert t \vert)s$. This is
    not standard.
 - Line 127. Use "... p \pi^0$ for 2.0 ...".
 - Line 129. Again, this odd notation for the cross section appears again.
 - Line 132. Use "... range of exclusive ...".
 - Line 140. Your units here should be GeV not MeV.
 - Line 147. Use "$E = 2$~GeV".
 - Line 151. Use "... the CLAS detector at Jefferson Laboratory using a ...". Be sure to
    include the CLAS reference here (B.A. Mecking {\it et al.} ...}.
 - Line 152. Use "... from a 5.72~GeV ...".
 - Line 155. Use "$g12$".
 - Line 159. Use "... decay into an ...".
 - Line 161. What do you mean by "$\pi^0 \to \gamma^* \gamma$"?
 - Line 163. Use "... tracks, $p$, $e^+$, $e^-$, as ...".
 - Line 164. Use "... which imposed limitations ...".
 - Line 169. Use "... for $p$, $\pi^+$, and $\pi^-$ tracks, ...".
 - Line 172. The statement of "standard $g12$ calibration" is not meaningful for the reader.
   A reference is necessary here.
 - Line 173. Put [21] at the end of this sentence.
 - Line 181. Use "mass of the $e^+e^-(\gamma)$ to be the $\pi^0$ mass.".
 - Line 182. Use "... using the statistical ...".
 - Line 185. Do not begin a new paragraph with this sentence. It belongs with the previous
    paragraph.
 - Line 186. Use "... between the $g12$ ...".
 - Line 194. Use "... off the proton, $M_x^2(p) = (P_\gamma + P_p - P'_p)^2$, in terms of the
    four-momenta ...".

Page 3:
 - Fig. 1:
   - The plots need to have the color z-scale include.
   - Caption.
     - Line 1. Use "(Color online) (Left panel) $M_E(pe^+e^-)$ vs. $M_x^2(p)$.
       (Right panel) M_x^2(pe^+e^-)$ vs. $M_x^2(p)$. The top-right (bottom-right)
       panel shows the distribution before (after) applying the $M_E(pe^+e^-) < (>) 
       75$~MeV cut.".
 - Fig. 2 caption.
   - Line 2. Use "... in the proton missing mass squared for events ...".
 - Line 195. Use "... proton, and final ...".
 - Line 196. Use "... proton, respectively, and the ...".
 - Line 198. Use "... the $\pi^+\pi^-$ ...".
 - Line 206. Use "... by the red solid line.".
 - Line 207. Use "... 0.0179$~GeV$^2$ with a Gaussian width ...".
 - Line 208. Use "... asymmetric cut about the measured value was placed ...".
 - Line 210. Include units on mass squared range.
 - Line 216. "Overall the systematic uncertainty varied ..." Systematic uncertainty
   on what quantity? Be clear.
 - Line 221. Use "$z$-vertex".
 - Line 222. What do you mean by the systematic uncertainty on the "sector-to-sector"?
 - Line 224. What do you mean by the systematic uncertainty on the "1C pull probability"?
 - Line 227. Use "The new CLAS high statistics $\gamma p \to \pi^0 p$ cross sections from
    this analysis are compared in Figs.~3 and 4 with data from previous CLAS~[19], DESY
    [add ref.], CEA~[add ref.], SLAC~[add ref.], and Cornell~[add ref.] measurements, as
    well as lower c.m. energy MAMI A2 measurements~[26]."
 - Line 238. Use "... scaling at fixed $t/s$ as expected ...". This is also a bit of jargon
    that the non-expert will not understand.
 - Line 241. Use "... leading to $n = 6.89\pm0.26$."

Page 4:
 - Fig. 3 caption.
   - Line 1. Use "... cross sections for $\gamma p ...". Write cross section in 
     math mode.
   - Line 3. Use "... tagged photon data ...".
   - Line 4. Use "The black open filled ...".
   - Line 5. Use "$E$ = 2~GeV [18]. The plotted ...".
   - Line 6. Use "The black dot-dashed ...".
   - Line 7. Use "... with $n = 6.89 \pm 0.26$, ... The pion production ...".
   - Line 8. Use "... The Regge results [4,9] are given by the black dotted and
     blue dash-dotted lines, respectively.".
   - General: The Regge results are very difficult to even find on these plots.
 - Line 243. Put "s" in math mode.
 - Line 249. Do not begin a new paragraph with this line. It belongs with the
   previous sentence.
 - Line 251. Put "E" in math mode.
 - Line 253. What is the angle theta? When it is first introduced make this clear.
   If it is the pi0 CM angle, I would suggest using throughout "$\cos \theta_{\pi}^*$".
 - Line 256. Use "... of these dips. Note that ...".
 - Line 259. Put "E" in math mode.
 - Line 260. Awkward phrasing. I suggest "... 2.6~GeV$^2$ and evidence of structure
   around ...".
 - Line 262. Put "E" in math mode. 
 - Line 268. Use "... at the largest $\vert t \vert$ ...".
 - Line 275. This paragraph is a bit of a misplaced orphan. Move it to the end of the
   paragraph beginning on line 281.
 - Line 282. Use "... based on the $\pi^0$ Dalitz decay mode.".

Page 5:
 - Fig. 4 caption.
   - The energy variables "E" and "W" in the caption (lines 2, 3, 4, 6) should be 
     written in math mode, i.e. "$E$" and "$W$".
   - Line 4. Use "... CLAS $g12$ measurements (red ...".
   - Line 7. "... indicated on each panel.". There are no energy values indicated on
     any of the panels.  
 - Line 290. Use "... covered by the "resonance" ...".
 - Line 293. Use "... here have quadrupled ...".
 - Line 294. Use "... above $E$ = 2~GeV ...".
 - Line 296. The main conclusion sentence "By comparing this new and greatly ..."
   does not make any physics statement. This needs some attention as it is far from
   satisfying.
 - Line 302. Use "Ph.D.".

Page 6:
 - Ref.[7]. Add a comma after "Ostrick".
 - Ref.[18]. Remove extra space before final period.
 - Ref.[19]. Add "[CLAS Collaboration]".
 - Ref.[23]. Remove extra space after "ACAT".