[Clascomment] OPT-IN:Measurement of the generalized form factors near threshold via gamma*p --> n pi+ at high Q^2

[Daniel Carman]

					November 17, 2011


Dear Kijun and Ralf,

I have read through your npi+ form factor paper and include my comments below. 
The paper is in reasonable shape but I did have a few analysis questions as
I did my reading and a list of grammar/style/wording issues that I found in
my reading. Let me know if you have any questions. Oh, and thanks for including
the line numbers in the draft, it makes it easier to communicate my comments.
Note that these numbers don't always make perfect sense on each page, but I
still refer to them in my notation below.


			   Regards,

				  Daniel

********************************************************************
Page 1:
 Abtract.
  Line 5. I suggest "In this work we analyzed a data set that utilized a
    high-energy electron beam on a proton target using the nearly $4\pi$
    CLAS detector.".
  Line 10. I suggest "We also compare measurements of $G_1$ and $G_A$ with
    recent models.". 
 Left column.
  Line 10. Use "photo-".
  Line 12. I suggest "These studies are of interest because the ...".
  Line 13. Use "... chiral symmetry, supplemented ...".
  Line 14. Use "... current algebra, allows exact predictions to be made for ...".
  Line 19. Use "cannot".
  Line 40. Use "... that includes CHPT ...".

Page 2:
 Right column.
  Jacobian Equation - E_e and E_f are not defined in the text.
  Line 66. Use "... photon polarization and $\Gamma_v$ is ...".

Page 3:
 Left column.
  Line 12. Use "$d$-waves".
 Right column.
  Line 1. Use "... by the dipole form factor ...".

Page 4:
 Left column.
  Line 12. Use "... Spectrometer (CLAS) ...".
  Line 21. Use "Cerenkov" or better yet "Cherenkov". Fix all other instances in
   paper.
  Line 22. Use "... for electron/pion separation ...".
  Line 24. Use "... neutron detection, as well as triggering.".
 Right column.
  Fig. 1 caption.
   Line 2. I suggest "The top panel shows a horizontal cut through the beam line.".

Page 5:
 Left column.
  Line 4. Use "e1-6 run period".
  Line 7. The listing of currents for the torus and mini-torus is not meaningful
   to non-CLAS folks. Better to list the integral b x dl for the magnets.
  Table I caption.
   Line 2. Use "... $\theta_\pi^*$ and $\phi_\pi^*$, the polar and azimuthal ...".
   Line 4. Use "... and $Q^2$ the four momentum ...".
  Line 28. I suggest "After applying our particle identification (PID) and
    kinematic corrections for the near threshold regime, the total number ...".
  Line 32. The sentence "Since PID and kinematic corrections have a strong
    dependence on event statistics, our PID and corrections ...". Actually, I
    am not sure what you are trying to say. I think you are saying that having
    high statistics improves the ability to set your cuts and determine your
    corrections, as well as to study the associated systematics. Am I correct? 
  Line 38. Here (and at several places in the rest of the paper) you refer to
    "the previous analysis" (Ref. 25). What is this analysis and why is it
    relevant. Certainly the first time that you mention Ref. 25 you should clearly
    state its relevance.
 Right column.
  Fig. 2 caption.
   Line 3. Use "... $W$ (top) and $\phi_pi^*$ versus ...".
   Line 61. The EC sampling fraction actually has a moderate dependence on
    electron momenta for low momenta. I think it is more appropriate to say that
    the ratio is nearly momentum independent for the range of electron momenta 
    (2.5 to 4 GeV) in this analysis.
   Line 1. The difference in sampling fraction between data and MC is kind of a
    known difference that must be taken into account. However, with more information,
    this difference does not look so good to an outside reader. Isn't the issue
    with the MC simply the fact that the EC simulation is just a toy model of the
    shower production? Perhaps it is better not to mention the MC sampling fraction
    value specifically, but just mention that the MC value was found and a +/-3sig
    cut was used here as well.

Page 6:
 Left column.
   Line 6. I prefer "$\pm 3 \sigma$" (here and in the Fig. 3 caption).
   Fig. 2. Typo on plot label, "exclusivity". Also, the cuts on the data (left plot)
    look odd to me. Why isn't are the curves symmetric on the data?
   Line 10. Use "EC" instead of spelling out electromagnetic calorimeter again.
   Line 19. I suggest that you drop the sentence "The calorimeter threshold
    did not depend on beam energy." It adds nothing of value here and just confuses
    the discussion.
   Line 35. Here you use "DC" notation, but I did not see it defined back at the
    start of Section IV.
   Line 41. I think you have spent too much time talking about the vertex cuts. The
    cut is pretty simple and easily justified with a sentence or two. If you wish
    to keep what you have, I won't object, but the context is not so meaningful
    without stating what the vertex resolution of CLAS actually is. You have a
    5-cm-long cell, but you have a much wider cut. I don't know what to make of the
    offsets without knowing about the beam size at the target and the transverse
    size of the target cell.
 Right column.
   Line 43. Use "$z$-vertex".
   Line 46. Use "We corrected the $x$, $y$ vertices ...".
   Line 65. Use "... in the CC sectors".
   Line 68. Isn't your NPHE cut really at 2.5? The nt10 variable is multiplied
    by 10 isn't it?
   Fig. 4. You mention different line-type thicknesses for the left plot. The red
    and black line types look the same to me.

Page 7:
 Left column.
  Line 2. You mean "TOF system" here, not drift chamber.
  Line 9. Use "$\pm 2 \sigma$".  
  Line 9. Also nobody knows what SEB is. In fact, this last sentence provides no
   useful information. I suggest that you drop it entirely.
  Line 15. Use "... was applied to $\beta_h$ ...".
  Line 17. Use "scatterplots" or "scatter-plots".
  Line 19. Here you mention "remaining positrons". This comes out of nowhere. What
   positrons?
  Line 33. Use "... and high reconstruction efficiency.".
  Line 37. Provide a statement as to what GSIM is.
  Line 42. Use "... momentum, and the strength of the ...".
 Right column.
  Line 53. Use "... uniform and large efficiency.".
  Line 62. The sentence detailing that your cut function depends on B/Bmax is kind
   of meaningless to an outsider. I suggest that you drop this sentence altogether.
  Line 66. What is a "centroidal electron angle"?

Page 8:
 Left column.
  Fig. 6. 
    On the left plot, the polar angle range is not appropriate. It goes out
    to 80 deg. This should be limited at least to the physical range of the detector.
    This would improve the presentation because your data are all bunched up on the
    left side of the plot.
    Line 8. I suggest "The right plots shows examples of the $\phi_e$ ...".
  Line 14. Use "... the drift chambers.".
 Right column.
  General. 
   You did not mention that you account for the DAQ/triggering live time.
  Section C.
   This section starts off kind of awkwardly. There is no context for the
   "kinematic corrections" that you mention. Corrections for what quantities caused
   by what effects?
  Line 1. Use "phase space".

Page 9:
 Left column.
  Fig. 8 caption. 
   Line 2. Use "... Gaussian function in the bin at $\cos \theta ...".
   Line 14. Use "$W$".
   Line 15. Use "$Q^2$".
   Line 16. Use "... data sets were ...".
   Line 29. GPP stands for "GSIM post-processor".
   Line 29. Actually I find this who paragraph awkwardly written. I suggest
    "The GSIM Post-processor (GPP) is used for fine adjustments of the
     reconstructed GSIM data to better match the measured data. GSIM simulates
     events in an ideal detector system and GPP is used to adjust two quantities.
     One is the drift chamber position resolution smearing factors that affect
     the tracking momentum resolution. The other is the TOF time smearing factor
     that affects the timing resolution. The optimized GPP settings were taken from
     Ref.[25].".
 Right column.
  Fig. 9.
   The vertical plot labels overlap with the numbers.
   Caption.
    Line 5. Use "... without the GPP step.".
    Line 6. Use "... on the top-left plot ...".
   Acceptances. You do not mention anything on the statistical uncertainties on
    the determined acceptance functions. I should think a statement on the range
    of uncertainties across your kinematic space is appropriate in relation to
    the overall statistical uncertainties on the cross sections and structure
    functions. I assume that these uncertainties are accounted for in your error
    bars?


Page 10:
 Left column.
  Line 9. Use "... to events without radiative effects in a given kinematic bin.".
  Line 15. I don't know what you are talking about when you mention "integrated
   luminosity".
  Line 16. Use "... non-radiative events, respectively."
  Line 27. Use "... between the two models, ...".
 Right column.
  Line 42. How was this minimum acceptance value chosen? It seems quite arbitrary.
   Did you study this as a source of systematic uncertainty?
  Line 48. Use "... Taipai (DMT) ...".
  Line 54. I think the sentence "Table II summarizes the ..." really starts in on
   a whole new discussion. This sentence should begin a new paragraph.
  Line 59. I suggest "... centering effects, the particle identification (PID) for 
    electrons and pions, and the different physics event generator calculation of
    the acceptances.".

Page 11:
 Left column.
  Fig. 12 caption.
    I prefer "uncertainties" to "errors".
  Line just before Eq.6. Notation change. Use "$\sigma_T + \epsilon \sigma_L".
  Line 6. Use "Figures 14 and 15 ...".
  Line 8. Use "... shown, several features are notable.".
 Right column.
  Table II. How is the CC efficiency handled? It is non-zero over a notably smaller
   region that your defined electron fiducial cuts.

Page 12:
 Left column.
  Fig. 13. I notice that there appears to be an somewhat abrupt transition in
   the three middle Q2 bins for W=1.1 Gev precisely where you transition phi bin
   size. This makes me a bit concerned on the analysis side, especially as no
   systematics are assigned to the extraction associated with more or less phi
   bins. As you reduce the number of phi bins, you can lose sensitivity to
   cos 2phi, which feeds into your constant and cos phi terms. Have you studied
   how your extraction of the structure functions depends on the number of phi
   bins in the fit?
  Line 41. Use "distributions of ...".
  Line 42. Use "... As mentioned in Section IIIa, ...".
  Line 3. You switch notations between "$\sigma_T + \epsilon \sigma_L$" and
   "($\sigma_T + \epsilon \sigma_L$)". Choose one and stick with it.
  Line 17. Use "... whereas the MAID models predict a sizable ...".
 Right column.

Page 13:
 Left column.
   Line 6. Use "1.11 GeV".

Page 14:
 Left column.
  Fig. 17 caption.
   Line 4. I prefer "uncertainties" to "errors". Same comment in line 7.
  Fig. 18 caption.
   Line 4. Use "... squares: Platchkov [32] ...".
   Line 6. Use "... triangles: CLAS measurement ...".
 Right column.
   Line 19. Don't add unnecessary notation changes with "$\sigma_{T + L}$". Stay
    with "$\sigma_T + \epsilon \sigma_L$".

Page 15:
 Left column.
  Fig. 20 caption.
   You have changed your labeling here. I suggest "$Q^2$ dependence for
    $n\pi^+$ of $G_1$ normalized by ...".
 Right column.
  Line 33. Add a period at the end of the sentence.
 References.
  Put the references in the order in which they are cited in the paper.