[Frost] Analysis review comments

[Michael Dugger]

Hi,

We have received comments from the Analysis Review Committee regarding the 
helicity asymmetry for eta photoproduction.

As discussed during today's FROST meeting, some of the comments we received 
might come up again for others during an analysis review of g9a data.

At the bottom of this email I have copied those committee comments that 
are not specific to the eta analysis. I have placed, what I feel is the 
most important committee comment first. This comment deals with the fact 
that carbon is not helium or oxygen, and is a question I expect all 
analysis reviews will want a good answer for. Any help in addressing the 
comment regarding the differences between carbon and helium and oxygen is 
greatly appreciated. In fact, I feel it would be good that the FROST group 
develop a standard answer to this.

Thanks for your time.

Sincerely,
Michael

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Committee comment->


Assumption A. Nuclei have Fermi momenta and excited states. These
additional degrees of freedom can change the momentum balance and
result in negative tails on an M^_x distributions calculated with
free nucleon kinematics. The shape of the M^2_x distributions, and
in particular the fraction of events in the negative tail M^2_x <
-0.4 GeV^2, is assumed to be the same for events originating on
any nucleus. In particular, region (1) contains 3He and 4He (as
coolant in the FROST mixing chamber), 12C and 16O (in the
Butanol), 19F, 35Cl and 37Cl (in the pCTFE target cup), while
region (2) contains 12C. We would expect the general shapes of
M^f_x distributions from quasi-free production on these nuclei to
be similar, particularly near the quasi-free peak, but the tails
depend on how the 2-body kinematics is miss-matched by Fermi
motion and the spectra of nuclear excited states in the recoiling
product nuclei. It would be amazing if the tails were identical!
We can assume the above equation for the moment, but there needs
to be some reasonable (justifiable) estimate for the systematic
error associated with this assumption.

Response->

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Committee comment->

p1, Section II:
There are 2 different conventions in use for defining the net
entrance channel helicity which differ by a sign, either the
scalar sum of the particle helicities in the center of mass, or
what amounts to the net projection of the total angular momentum
along the beam axis. Judging from Section III.F, the authors are
using the latter. It would help to avoid confusion if the first
equation were augmented to include something like,

E = [H_{1/2} - H_{3/2}]/[H_{1/2} - H_{3/2}]
   = [\sigma_{1/2} - \sigma_{3/2}]/[\sigma_{1/2} - \sigma_{3/2}]

where the A and P designations refer to antiparallel and parallel
beam and target spin alignments. With this there can be no
confusion. There are also different definitions of E  in terms
of observable quantities, which differ by signs. The authors have
chosen a particular one. A translation table relating different
schemes in use in the literature is given in arXiv-1108.5411.
Since the assignment of helicity states is referenced to \pi^+
production in III.F, it would be helpful to specify the
convention.

Response->

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Committee comment->

p1, Setion III:
The text states that "energy and momentum corrections" are
applied to the proton. Technically, correcting one of these
automatically corrects the other. Typically, CLAS analyses
correct the "momentum" of the detected particles and the "energy"
of the photon beam. The energy of the proton can't really be
"corrected" , since its energy is never measured. Rather, because
it looses energy passing through the material within CLAS, the
measured momentum isnt correct and must be adjusted. Further,
note that the kinematic fitting routine does not "correct" the
energy; rather, it applies the known measurement uncertainties to
determine the most likely "actual" value of a measured quantity,
using what amounts to an economic algorithm.

Response->
We have removed the mention of energy corrections.


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Committee comment->

Section III.A:
It would be useful to remind the reader at the outset of the
composition of the target region involved in the analysis. In
particular, instead of a reference to "FROST", it would have been
useful to clearly state the composition as 5 cm of Butanol
(C4H9OH) centered at a z-vertex position of _____, __cm of Carbon
centered at z = ____cm, and ___cm of CH2 centered at z = _____cm,
etc.

Response->
We have included the requested text that the 5.28 cm butanol (C4H9OH) 
target is centered at a z-vertex position of 0, 0.15 cm of carbon is
centered at z = 6.25 cm, and 0.35 cm of CH2 is centered at z = 16.1 cm.

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Committee comment->

p2, Section III.B:
What was the range of the live-time during the runs? There are
typically some runs with abnormally low live-time, due to
beam-steering, etc, but these seem to have been included.

Response->


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Committee comment->

p3, Section III.C:
This is a single-hit analysis whenever more than one tagger
channel fires within an RF bucket (1 ns), the event is discarded.
A more general way to treat such multi-tag events is to update
each of the histograms associated with those energies for which
tagging channels fired within the true timing peak; multiple
over-counting is then corrected at the end by an accidental
subtraction. This is a little more work, although not all that
much, and potentially improves statistics. The improvement might
be negligible if the fraction of multiple tags is small. Since the
statistical errors on the extracted asymmetries are appreciable
for some bins, this should be addressed  perhaps with a plot of
the multiplicity in the tagger, or at least a statement about the
relative fractions

Response->
As noted, there is two ways of dealing with multiple tagger hits
for an idividual event: Either throw away hits that are ambiguous,
or loop over each possible photon. We have always chosen
to throw out ambiguous photon events so as to keep the signal as
clean as possible (Differential cross sections using g1 data for
eta, etaPrime, pi0, pi+, as well as for beam asymmetries using
g8b data for eta, etaPrime, pi+, pi0).

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Committee comment->

p5, Section III.F
The discussion of identifying beam helicity states is confusing.
The text states that "the helicity 3/2 state was assigned a
negative sign" based on \pi^+ production. I presume this means
that the E asymmetry for single \pi^+ production was assumed to
be negative near 900, which it would for
E = (\sigma_A-\sigma_P)/(\sigma_A+\sigma_P). That fixes the beam
helicity states and those assignments were use in the
(\gamma,\eta) analysis. Please clarify the wording.

Response->

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