[Gdh_lowq2] Your_manuscript CU10463 Zheng

Tue Aug 16 15:58:00 EDT 2016

Dear All:

I have started looking into these comments and I can use some help, as follows:

1) (Fig.6) Does anyone recall what happened to the beam polarization during the 1.3 GeV NH3 run period? The Moller polarization jumps up and down quite a bit and they were definitely not statistical fluctuations.  Did the beam change a lot (that is, at least 4 times) during that period because other halls requested different beam energy, pass number, or polarization?

2) THe reference mentioned by the referee on the pion photoproduction data, PRL112(2014)012003 seems to show that A_LL (our definition, as 1/2-3/2) should be >0 for D13(1520) and <0 for F15(1680), while I think our data indicate that both are <0 in the lowest Q2 bin. Should I say something about the inconsistency with the photon data for the D13(1520)?   (Or, should I not comment on it because the observables in photo- and electro-production may not be a one-one mapping?)  I am also puzzled why the D13 has >0 sign (1/2-dominance) from the photoprod data, opposite to PDG(2000)?

For your convenience, the submitted version (or nearly) can be downloaded from http://arxiv.org/pdf/1607.03924.pdf (it is nearly identical to the version read by the referee. Differences did exist due to the PDF rendering processer on different websites, which explains the missing labels...)

More questions to come.  Thanks in advance!

Xiaochao

----- Original Message -----
From: "Xiaochao Zheng" <xiaochao at jlab.org>
To: "gdh lowq2" <gdh_lowq2 at jlab.org>
Sent: Thursday, August 4, 2016 3:29:23 PM
Subject: Fwd: Your_manuscript CU10463 Zheng

Hello all:

Below please find comments on the EG4 exclusive paper.

I will work with Marco on details of the cherenkov and then will work on all other comments as well.  Help will be appreciated in dealing with these comments, so please feel free to comment.

Thanks,

Xiaochao

----- Forwarded Message -----
From: prc at aps.org
To: "Xiaochao Zheng" <xiaochao at jlab.org>
Sent: Thursday, August 4, 2016 3:02:18 PM
Subject: Your_manuscript CU10463 Zheng

Re: CU10463
    Measurement of target and double-spin asymmetries for the  vec e vec p
    rightarrow e pi  +(n) reaction in the nucleon resonance region at low
    Q 2
    by X. Zheng, K. P. Adhikari, P. Bosted, et al.

Dear Dr. Zheng,

The above manuscript has been reviewed by one of our referees. Comments
from the report appear below.

These comments suggest that specific revisions of your manuscript are
in order. When you resubmit your manuscript, please include a summary
of the changes made and a succinct response to all recommendations or
criticisms contained in the report.

Yours sincerely,

Bradley Rubin
Senior Assistant Editor
Physical Review C
Email: prc at aps.org
http://journals.aps.org/prc/

COLOR REMINDER:
  You have requested to have some color figures in the print
  journal (5,9,10).  If your paper is accepted you will
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----------------------------------------------------------------------
Report of the Referee -- CU10463/Zheng
----------------------------------------------------------------------

The authors report about new measurements of pi+ electroproduction
with the CLAS spectrometer at JLab using a polarized beam and a
polarized target. The data cover the center-of-mass energy range of
the full nucleon resonance region in particular at very low values of
4-momentum transfer. This is valuable new information about the
transition of transverse amplitudes from photoproduction to finite
virtualities as well as about longitudinal amplitudes which cannot be
accessed with real photons.

In the paper no LT-separation is performed but the data are presented
in terms of target and beam-target asymmetries and are compared to
model calculations. The paper is well written and covers the relevant
experimental issues. I have no reservations on the main content, the
analysis procedures, the discussion of systematics and the
presentation of the results. A publication in PRC is fully
appropriate.

I suggest the following checks and modifications which would improve
to quality of the paper:

(1.) I propose to use the nomenclature for baryon resonances suggested
by PDG:

e.g. P33(1232) -> Delta(1232)3/2+ (similar for all states).

(2.) Page 3, line 159- 163:

The authors mention pion photoproduction data but refer only to
upcoming CLAS data. Here the published work of other labs should be
mentioned as well (e.g PRL 112 (2014) 012003, ...).

(3.) page 5

In Fig.2 the labeling inside the picture is missing. I see only the
arrows.

(4.) page 5, line 313-324.

The new Cerenkov counter is introduced but no details are given. I
assume that its structure and performance (gas, efficiency,..) are
similar to the existing Cerenkov detectors and Ref.35 is also relevant
for the new device. If this is correct one could make such a statement
or give some numbers and further reference should be given.

(5.) page 7, line 482

"Beam charge asymmetry ...." replace by: "A helicity dependence of the
integrated beam charge...."

(6.) page 8, Fig.6

I do not understand the interpretation of the data points and
extrapolation bands. What happened e.g. before run number 51500 (1.3
GeV data)? Two Moller measurements are significantly lower compared to
other. It should be explained why the inter-(extra-)polation was done
as indicated by the gray bands. I assume, that its due to changes in
the configuration or breaks during data taking. Same question comes up
when looking at the extrapolation of the last 2 data points.

(7.) page 8, Fig.7

axis labels missing

(8.) page 8, line 549

"ep -> e'pi+(n)" replace by "ep -> e'pi+(X)" as correctly done in the
caption of Fig.8

(9.) page 9, caption of Fig 8.

.... (the proton mass) replace by .... (the neutron mass).

(10.) page 9, eq.(26) and eq.(27)

There are more recent form-factor data at low Q2 which where not
available in 1995 when the fit of Ref.(45) was performed. I assume
that details in the form-factor parametrization at low Q2 are not very
important for the method and additional errors from uncertainties in
our knowledge on proton form factors are small. Probably even a simple
dipole parametrization would work. If my assumptions are correct, a
short statement about the sensitivity of the method to the form factor
parametrization should be added.

(11.) page 10, Fig.9

The variable "dil" is not defined. I would use the name "f_el^incl" as
in the text.

(12.) page 9, line 578-580

A reference for the formula A^el_th = ... should be given. I think
that in the second term of the denominator G_M^p should be replaced by
(G_M^p)^2.

(13.) page 8/9

In the description of the reconstruction of elastic events the authors
refer to an invariant mass (e.g. page 9 line 584). To be consistent
with the other chapters, this quantity should be called "missing
mass".

(14.) page 10, line 675

The authors claim that their run-by-run values of Pb*Pt determined via
elastic scattering are consistent with all configuration changes. I
propose to include a figure similar to Fig.6 where Pb*Pt is shown as
function of the run number (or blocks of runs). Such a figure would be
helpful for the reader to get an impression about the data taking and
the consistency.

(15.) page 11, section E.

It should be made clear that the discussion about the asymmetry
extraction is only valid in each single kinematic bin
(Q^2,W,theta,Phi). Only later in the text it becomes clear that this
was correctly done. I propose to introduce the 4-dimensional grid of
the kinematic variables used to extract the asymmetries already at
this point.

(16.) page 13, Fig. 10

I assume that the missing mass distributions are integrated over all
other kinematic variables. I suggest to show also the missing mass
spectra for e.g. 2 typical kinematic bins, which have been used in the
real analysis.

Replace the label "dil" in the figure by "f_dil^pi" as given in the
text.

(17.) Presentation of results

The way the results are presented and compared to model calculations
is reasonable for this experimental paper. I have two suggestions:

a) The authors stress several times that the new data are consistent
with previous data at higher Q2. But no comparison is shown. I propose
to include a plot which shows the consistency to previous data or to
add previous data in existing plots.

b) Results for the beam asymmetry A_LU were obtained but are not
discussed in the paper.

The authors say that A_LU was just used as a cross-check and that the
results are available for download. This is reasonable as this
observable can be better obtained with a hydrogen target. Nevertheless
a short comments about these data should be made (E.g. are the data
consistent with existing data? What is the contribution of quasi-free
events?). Maybe a reference (thesis?) could be added.