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

Tue Nov 29 04:16:17 EST 2011

Dear authors, 

Congratulations for this nice draft, revealing a huge analysis work and 
interesting results!

GENERAL COMMENTS:

The text, however, still needs substantial improvements:
- The abstract contains several repetitions and inaccuracies and, in my 
  opinion, needs to be completely rewritten.
- The text contains some redundancies and missing or misplaced 
  definitions
- I would skip "(Color online)" in the figure captions
- labels and scales in plots are ofter too small or too light

ABSTRACT:

lines 1-2: you claim your results are obtained "at high momentum transfer". 
           "High" is, of course, relative (i.e. other experiments can reach 
           much higher Q2). So you'd better report the actual Q2 range for
           this analysis or the upper limit ("Q2 up to 4.2 GeV^2").

line 6: "recent experimental data of CLAS". This is in conflict with what 
         is written in pages 4-5, where you claim that these data was  
         collected from Oct. 2001 to Jan. 2002... i.e. 10 years ago.

line 7:  "high energy electron beam"... again, this is relative. 
         The actual beam energy for these data is 5.754 GeV (as you claim at 
         the beginning of page 5), which is for sure not high (if compared 
         with other experiments in the field)

I propose to rewrite the abstract as follows:

"We report the first extraction of the pion-nucleon multipoles near the 
production threshold for the npi+ channel at relatively high momentum transfer 
(Q^2 up to 4.2 GeV^2). The dominance of the s-wave transverse multipole (E0+), 
expected in this region, allowed us to access the generalized form factor G1
within the light-cone sum rule (LCSR) framework as well as the axial
form factor G_A. The data analyzed in this work were collected by the nearly 
4pi CEBAF Large acceptance Spectrometer (CLAS) using a 5.754 GeV electron beam 
on a proton target. The differential cross section and the pi-N-multipole 
E0+/G_D were measured using two different methods, the LCSR and a direct 
multipole fit. The results form the two methods are found to be consistent and 
almost Q2 independent.

DETAILED COMMENTS ON THE TEXT:

PAGE 1

line 12: skip comma after "interesting"

line 15: "known as" -> "based on the so-called"

line 48: "high energy electron beam" -> 
         "5.754 GeV electron beam" or "nearly 6 GeV electron beam"

line 50: "allows" -> "allowed"

line 62: skip comma after Q2

line 63: "small, but in this case," -> "small but, in this case,"

line 72: "carried out SO FAR"

PAGE 2

line 18: "with Q2) and nonperturbative" -> "with Q2), and THE non-perturbative"

lines 31-32: skip commas before and after "because in LCRS"

line 53: "kinematic threshold" -> "threshold kinematics"

Add eqn. numbers in the equations in top-right of page 2

lines 65-81: this paragraph is ment to describe the equations above.
             However, the definitions of some of the variables appearing in the 
             equations are not given (e.g. W, E_f, nu, Theta_e Omega_pi^*) whereas 
             the definitions of other variables are provided (p_pi^*, theta_pi^*, 
             k_gamma^*) which are not used in the equations above. So, please add
             the missing definitions and postpone those that are not needed here.  
             Also the meaning of the index "u" in "sigma_u" is missing. 

line 72: "polarization of beam" -> "polarization of the beam electron"

line 82: put here (after "where") the definition of theta_pi^* given at line 67

PAGE 3

Equation 2: k_i and K_f are not defined in the text

lines 6-8: rephrase: "initial proton), c=sqrt(2) is the isospin factor, f_pi=93 
           MeV is the pion decay constant, and g_A=1.267 is the axial coupling."

line 25: skip comma after "G2"

line 26: add comma after "threshold"

line 27: "npi+ reaction." -> "npi+ channel:"

Add full stop after eq. (4)

28: "Because" -> "since"

line 2, right: "described by" -> "described in terms of"

line 22, right: skip "then"

line 30, right: avoid repetition of "since": "(since G_E^n"  -> "being G_E^n" 

PAGE 4

line 2: add semicolon after "0.71)" and full dot after eq. 5

line 12: separate "Spectrometer" form "(CLAS)"

line 14: skip "distribution"

line 22: "electron and pion separation" -> "electron/pion separation"

line 2, right: skip "that we"

PAGE 5

line 13: "contributed only a fraction to the total" ->
         "contributed to only a small fraction of the total"

line 20: "table I shows" -> "table I reports"

line 21: "number of bins FOR THE RELEVANT VARIABLES"

TABLE I: the caption should not contain the definition of the variables. 
         Instead, these definitions should be given in the text at the proper 
         locations. For instance this is the first place where I encouter the 
         definition of W, which should appear right after the equations in the 
         top-right corner of page 2. Also, the definition of q and the relation 
         Q2=-q2, which are basic, should be given at the very beginning, in the 
         introduction.

line 29: skip comma and add "the" before "standard"

line 34: "reactions are investigated without the W..."
         "reactions were investigated before applying the W..."

line 46: skip "to select a good electron". This is slang.

line 52: "the remaining pion contamination by applying cuts" -> 
         -> "the residual pion contamination."

PAGE 6

line 9: "most of the pions produced in this experiment" ->
        "most of the produced pions"

line 11: "minimum ionizing PARTICLES"

lines 33-34: skip whole sentence since redundant

line 41: skip "for this experiment" to avoid repetition with next sentence

line 46: "x,y vertices for" -> "x-, y-vertex positions"

FIG. 4: from the plot it is not clear which line is "thin solid" and which is
        "bold solid". Use "black" and "red" or use a different line style.

PAGE 7

line 6: what is "(+)" ? Skip.

line 18: maybe there is too much space betwen "figure" and "5". Check.

lines 23-25: you speak about proton contamination, but you don't mention other
             hadrons, e.g. kaons. What about the kaons contamination?

lines 47-53: skip this whole part as totally redundant with previous paragraph 
             (lines 30-45)

lines 62-65: "The cut ... detector symmetry". This sentence is completely unclear
             to me. Please explain better.

PAGE 8

lines 10-11: skip sentence. This sentence is already provided in the correct 
             position at lines 19-20.

line 21: "by THE pion momentum"

line 22: "and THE angles theta_pi AND phi_pi"

line 35: "The correction is" -> "The corrections are"

PAGE 9

Fig. 8: Caption, second line: "function in a given cos(theta_pi) = 0.1 at W=1.11" 
        -> "function at cos(theta_pi) = 0.1, W=1.11"

line 32: "but SINCE the real CLAS"

line 37: "GPP process. One is the DC" -> "GPP process: the DC"

line 38: "and the other is the TOF" -> "and the TOF"

line 39: "adjusts THE timing"

line 48: "To calculate the acceptance of the pi+n reaction" ->
         "To calculate the acceptance for the pi+n channel"

PAGE 10

FIG. 10, caption, second line: "the gamma* p -> npi+ at a given W ..." ->
         "the gamma* p -> npi+ reaction at W ..."

line 6: "exact calculation". What that means? At which order of QED?
        Maybe better to use "estimate".

line 49: "MAID2003 [30] MODELS"

line 56: "13-  15" -> "13-15"

line 59: "effects and the particle" -> "effects, the particle"

lines 59-60: "electrons, pions and" -> "electrons/pions separation and"

PAGE 11

First paragraph of section B, second line: "obtain" -> "access"

First paragraph of section B, third line: "which corresponds" -> "corresponding"

First paragraph of section B, fourth line: "sigma_TT such that" -> "sigma_TT:"

lines 10-11: anticipate "plays a dominant role" before "(compare between ...)"

lines 12-14: "angles, and the lower ... without E0+." -> 
             "angles, especially at low Q2." 

line 16: "except FOR Q2=2.05"

line 16: "data are at a" -> "data exhibit a"

line 19: "which is THE closest to THE threshold"

line 20: "d-wave CONTRIBUTION"

line 28: "Figure 15. sigma_TT" -> "Figure 15. The structure function sigma_TT"
         (better not to start a sentence with a symbol)

line 40: "extracted by FITTING the cos"

PAGE 12

FIG. 13: caption, third line: attach semicolon to "predictions"

FIG. 13: the systematic errors often show large fluctuations in adjacent bins, 
         which sounds a bit odd. A linear fit (slope) of the systematic errors 
         would help to smooth these jumps, yet preserving the overal magnitude 
         of the systematic errors.

line 41: "and signma_TT STRUCTURE FUNCTIONS"

line 43: "(SEE Eq. (1))"

line 8: add comma between "terms" and "particularly" 

line 20: "Once we obtain the Legendre moments from ... can be directly"
         -> "The extracted Legendre moments can be direclty"

line 26: "nearest TO THE threshold"

line 32: "are presented" -> "are also shown"

PAGE 13

line 43: "lead to" -> "leads to"

line 15: "Here, theta_pi^* is the polar ...". Skip. It is already defined at Pag. 2.

line 17: "expressed by" -> "expressed in terms of"

PAGE 14

Fig. 17, Caption, third line: skip "that"

Fig. 18, caption, fourth and sixth lines: separate semicolons form following word

line 18: add semicolon at the end

Fig. 19: why this plot has a different scale than those of Figs. 17 and 18? 
         Better to use the same scale.