Draft informal notes from (gamma,p) meeting March 16, 2000...  RG

Present (at least sometimes): 
Chen, de Jager, Dieterich, Gayou, Gilman, Holt, Jiang, Jones, 
Kumbartzki, Malov, Meekins, Mitchell, Nappa, Perdrisat, 
Punjabi, Radyushkin, Ransome, Schulte, Strauch, Wijesooriya, ???

1) RG informally welcomed everyone.

2) Kees de Jager mentioned:
   a) We must give JLab seminar before publishing - Hall A requirement.
      Krishni is scheduleed for April 7.
   b) 2001 schedule: septum likely delayed. Polarized 3He tinstallation
      to start in March shutdown. New photodisintegration possible for
      February, if we can arrange consistent beam energy. But unlikely.

3) Steffen Strauch gave a dry run of his DPG talk. Some technical
   questions, no problems.

4) Steffen Strauch went over the helicity recovery problem. Code works
   down to few tens of Hz. Some ep and 1.3 GeV gamma d -> p n
   recovered. No problem foreseen with 2.5 GeV data, except for large
   number of runs and silo efficiency.

5) Olivier Gayou went over the ep analysis. Comparison with McNaughton and
   93-027 results for A_C, and Ge/Gm, good. "Photodisintegration" runs in 
   goodshape. Analyzing power has no problems in fixed energy / angle bins,
   but angle-averaged analyzing power can change, as efficiency vs angle 
   is affected at small angles (~3 degrees) by filtering, and at large   
   angles (20 or 30 to 70 degrees) by phase space of particles in focal
   plane. This only affects 3-70 average, not the 5-20 average. Also,
   for ~800 - 1000 MeV/c, small changes in energy seem to affect
   efficiency(theta) more than analyzing power(theta). Some runs still
   left to do. 

6) Krishni Wijesooriya covered many polarization extraction details
   - Final cuts in good shape, but not all data redone with final cuts.
     PID looks good, with improved databases. d/pi+ rejection looks good,
     and there is very little of either background.
   - Some questions about missing mass squared plots and what cuts were
     on VDCs. Krishni will make clearer.
   - p_x false asymmetry measurement looks good.
   - Studies of using energy specific vs. energy averaged false asymmetries
     for p_x, p_y show very little difference.
   - some question about number of events left after background subtraction,
     expect ~60% from raw spectra, but appears to be smaller in spectra
     with FPP cuts. Krishni will investigate further.
   - C_x, C_z look good, but helicity recovery replay at 2.5 GeV still to be
     done. Some other points also need final replay.
   - We did not take enough ep data for the highest energy points. Should have
     tried for 2-3 times the statistics in the ep. Data exist for 1.95 GeV
     but need to be replayed.
   - Full spin transport still needs to be done. RG opined it will make no
     difference.
   - Systematic uncertainties still need to be assessed. All studies so far
     indicate these will be at level << 0.05 we proposed. Uncertainty from
     p_y in ep, which is nonzero due to two-photon exchange, not well
     theoretcally constrained, in the range ~0 - 0.05.
   - RG mentioned want to still evaluate chi-squared for all azimuthal      
     distributions. See cos+sin terms give good fit for helicity differences.
     Question is for helicity sums: if cos + sin + cos(2phi) + sin(2phi),
     with latter taken from ep, do not give good chi square, what are
     implications for trusting induced polarizations?
   - After discussion, we decided it is better to finish analysis totally
     before publishing, rather than stick to arbitrary April 8 deadline
     and try to publish not quite final results. Even though all indications
     are that there will be no visible differences in the data.

7) Dave Meekins showed an extraction of lower energy photodisintegration cross
   sections. Plenty of statistics, questions about systematics. Used MCEEP to
   try to evaluate spectrometer solid angle. Gets agreement within ~20% of
   older data. Need to try to cross check with Xiaodong's acceptance model.
   Xiaodong's model is based on 15.5 degree calibrations from GDH experiment;
   extrapolation to 60 degrees unclear. Some tests will be done later this
   year.

8) Xiaodong showed some initial work on gamma p -> p pi0 cross sections.
   The problem in these data will be subtracting the ep radiative tail.
   The radiator changes the peak / tail shape, shifting and broadening.
   We will need to have ep event generation that fits radiator out, and
   radiator-in, "trusting" if it gets the peak that it also gets the tail
   right, both in number of events and in polarization.

9) Elaine Schulte showed the Hall C cross sections at higher energies.
   General impression that given the comparison, the Hall A lower energy
   spectra are quite clean, as Dave meekins has emphasized, and we do not
   need to worry about tails... The higher energy Hall C analysis has
   much fewer statistics, much more difficulty with PID, etc.

10) Roy Holt / Ron Gilman mentioned reactions to presentation of p_y.
   General surprise. Suggestions that more angles / energies needed. 500 MeV
   data very important towards making full set believable. Some suggestions
   from M. Rho about cloudy/chiral bag model. Almost noone believes pQCD.
   Radyushkin makes some informal comments, thinks real skewed parton
   calculation will be very difficult, but has some possible ideas.

11) Ron Ransome passes out draft article. Probably some changes in text needed
   as we consider final c_x, c_z values, and final numbers needed. We need
   to give collaborators about 2 weeks for comments, and then give CC 3
   "working" days to review final draft of paper, before submission.
   It would need to go out in next few days then to make April 7/8 deadline.

   Further discussion on other papers. There is intent to publish, e.g.,
   Phys Rev C brief report on the ep calibrations (Ge/Gm, and A_C). 93-027
   also needs to publish a long paper on the experiment. In addition, plan
   to publish NIM paper on FPP. Question is what data to show where, who is
   on author lists - if we show data all collaborators on paper, so some
   desire to have hardware description with ~no data and shorter author
   list on NIM paper. We agree to work out details in the "FPP working group"
   (about 80% of people at this meeting) meetings.

12) Tasks assigned for future work.
   KW: more details on gamma d -> p n polarizations
       start on gamma p -> p pi0 polarizations when done
   ES: gamma d -> p n cross section angular distributions; get initial
       help from Krishni for Hall A replay, help later from Dave and Xiaodong.
   DM: continue on gamma d -> pn 90 degree cross sections. Solid angle with
       Mark jones and Xiaodong.
   XJ: continue on gamma p -> p pi0 cross sections, get ep Monte Carlo from
       Andrei Afanasev