[Frost] Analysis review comments

[Igor Strakovsky]

Hi Mike,

Whois in the committee?

igor

On Thu, 9 Aug 2012 13:20:49 -0400 (EDT), Michael Dugger <dugger at jlab.org> 
wrote:

> 
> 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
> 
> ---------------------------------
> 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->
> 
> ---------------------------------
> 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->
> 
> ---------------------------------
> 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.
> 
> 
> ---------------------------------
> 
> 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.
> 
> ---------------------------------
> 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->
> 
> 
> ---------------------------------
> 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).
> 
> ---------------------------------
> 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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Igor Strakovsky, SAID CNS The George Washington University
Tel: 703-726-8344(NV),202-994-4742(FB),Skype: igors1945_2
Fax: 202-994-3001(FB),Emails: igor at va.gwu.edu, igor at jlab.org