Summary of the Hall D Workfest
Sept 10-12, 1998 at Indiana U

Updated: September 13, 1998

• Carnegie Mellon
  • Paul Eugenio and Curtis Meyer
• U Connecticut
  • Richard Jones
• Indiana
  • Alex Dzierba, Jeff Gunter, Craig Steffen, Eric Scott, Paul Smith, Craig Steffen, Adam Szczepaniak and Scott Teige
• JLAB
  • Joe Manak and Dennis Weygand
• North Carolina/Jlab
  • Andrei Afanasev
• ORNL/Tenn
  • Ted Barnes
• Rennselaer
  • John Cummings and Mina Nozar

Polarization Issues

Early discussion concentrated on the definition of polarization of light. Linear polarization is a coherent superposition of two circularly polarized states, why is there more information in linear polarization than in circular?

• In linear polarization, the polarization direction defines a NEW direction. In the case of circularly polarized light, the polarization direction is in the same direction as the photon, while in linear it is different: defines a plane
• Because linear light is a coherent sum of circularly polarized light, there are interference terms present that are not there for circularly polarized light.[There were dissenting opinions on this based on the statement that the interference is present ONLY because of the particular choice of basis vectors.]

Next we went through a discussion of the Schilling et al paper [On the Analysis of Vector-Meson Production by Polarized Photons by K. Schilling, P. Seyboth and G. Wolf, Nucl Phys B15 (1970) 397] . The main point, which is true independent of the produced meson system comes from Eqn. 31:

Where W_i are angular distributions based on the production mechanism, spin of the produced states, and angular momentum in the problem. Linear polarization gives us access to more observables, but W₁ and W₂ are separable into natural and unnatural parity exchange.

This led us to a discussion of generalizing this, and expanding things in terms of diagrams [1], [2] and [3]. Both [1] and [2] were t-channel exchanges. In [1] the baryon remained a nucleon, while in [2] the baryon was excited, and scattered against a spectator pion. Diagram [3] involved the s-channel formation of a baryon resonance. Discussion then concentrated on [1] and [2] and how are we going to fit the data using these. There are clearly issues of "double counting" involved in these, as each is a complete basis.

However, we have to input physics into the problem. We will only consider angular momentum up to some finite value. This leads to a truncated basis which will not completely span the space. It is highly unlikely that rapid phase motion in the meson system could be mimicked by one in the baryon system. The most likely situation would be that a resonance in one system leads to slowly varying phases in the other.

Other handles are the s and t dependence of the solutions. This is something that is not currently fully done in E852, but should be exploited. Finally, even though a finite sum of baryon amplitudes would have a projection into the meson system it is highly unlikely to actually improve the chi-square per degree of freedom of a fit. The bottom line is that there are many handles on this problem.

In one pion exchange, you learn nothing from polarization.

Richard Jones wrote a note on these issues - this will soon be posted on the web.

1. Extend the Schilling formalism to include a scalar and a vector particle in the final state. In this case, what does polarization tell us?
2. Examine the 3 diagrams and try to simulate some of the phase motion and reflections.
3. What would a polarized target do for E852? [Note from Alex: there is a discussion of this in two papers my M. Svec: Phys Rev D55 (1997) 4355 and Phys Rev D56 (1997) 4355 (yes - the page numbers are right). You can download the .ps or PRD versions of these papers by going to the PRD home page.
4. What is the staging issues for physics. How much software can be ready by the FSU meeting?

Tracking considerations for the FSU Meeting:

People/groups with a possible interest in the tracking:

• cmeyer@ernest.phys.cmu.edu [Curtis Meyer] CMU
• mueller@blakey.phyast.pitt.edu [Jim Mueller] Pitt
• mischke@lanl.gov [Dick Mischke] - LANL
• baraue@sarah.fiu.edu [Brian Raue] - FIU
• kramer@jlab.org [Laird Kramer] - FIU
• garcon@phnx7.saclay.cea.fr [Michel Garcon] - Saclay
• hcf@jlab.org [Howard Fenker] - JLAB

Curtis Meyer is in charge of coordinating tracking and will set up a web page. Issues include who is interested in building the various tracking chamers, working on electronics, etc...

Timetable and Goal of the FSU Meeting

The goal of the FSU meeting is to produce a design report - which should be in a form to present to the lab management and funding agencies sometime in January, 1998. Alex Dzierba will soon set up a skeleton version of the report on the web.

Monte Carlo Coordination

The code for the toy Monte Carlo was developed during the May Workfest at IU and is currently under code management at CEBAF using the cvs system.

Requirements for working on the code are a valid CEBAF user ID and that you know the password. All developent can be done locally.

In May, we developed two generators:

1. Scott Teige - cwrap - not yet checked in
2. Paul Eugenio - gener8 Available from CEBAF

Both produce 4-vector output in e852 "itape format".

1. Resolution Smearing functions were also generated. These were crude and need to be updated to address:
2. Resolution information for the Pb-Glass Wall.
3. Resolution for a Barrel Calorimeter.
4. What about conversions in the chambers?
5. Resolution information from the tracking information.
6. TOF information? Is this currently done sensibly?

Acceptance functions were also written. These will need to be improved as designs firm up.

All of these packages talk to each other via itape format. We also developed weighting functions for producing particular final states, and were able to pass the output into an early version of the spin-parity program.

What needs to be done?

1. Update of resolution smearing based on more accurate information.
2. Update of acceptance functions based on more accurate information.
3. A reconstrution piece is missing. This could start as simply putting photons back together as pio's and eta's , additionally it should include particle ID. Priority is a bit lower on this.
4. Weighting algorithems for the events. This needs to be developed including both beam polarization and potentially target polarization. This will hopefully be finsihed by FSU.
5. Beyond [4] is feeding everything into a PWA analysis. Time scale is a bit longer on this.

Identification of the 'Flagship' Physics for Stage-1

Exotic State: 1^{- +} state at 1.6 GeV decaying into rho-pi .

This state appears to be produced via rho exchange in E852. Nominally, it could be Pomeron, rho or f₂(1275) exchange. However, no f₂-pi decay is observed, meaning that this coupling is likely small. Secondly, the natuarality combined with the M of the exchange indicate that rho exchange is far more likely. This means that charge exchange (CEX) photo-production:

should be a very good way to produce this state.

It also appears that in photoproduction, the production of both the a_1(1260) and the pi(1300) are supressed, whereas in pi^- p , the difractive production of the a_1(1260) is very strong. This means that the leakage from the a_1 into other channels will be much smaller in photoproduction than in pi^- p . In particular, we have a much better chance of finding the 1.4 GeV 1^{- +} state in a rho pi decay if such exists. The rho pi channel should be much cleaner in this reaction.

Look here later for some discussion of the comparison of the photoproduction of 3-pi by Condo et al and a comparison with E852 results.

We also considered the production of rho-pi states with a recoil delta (or N*):

Advantages:

1. The trigger is relatively straightforward, in that it is two ~back to back tracks in the central region.
2. The process "tags" one pion exchange.
3. final state involves only charged particles.
4. In principal, we can determine the beam energy, so we may not need to tag.}
5. The process is one pion exchange, so polarization of the beam will add no new information. i.e. we do not need polarization for this.

Disadvantages:

1. How low can we go in t in this process? Nominally, this should be ok, but we need to produce a plot similar to those that we did last May. (Alex will do this
2. What about combinatorics? We have a couple of figures [from Scott] that indicate the combinatorics leads to something like a 10% background under the Delta. However, since we will have to may the amplitude symmetric under the exchange of identical pions, this should actually be built into the fit.
3. How do we do PWA with a Delta? This has apparently never been done, and does need some thought. However, our best estimate is that at very best, the analysis will be no worse than the normal nucleon process. Nominally the problem is rank 4 for the spin 3/2 Delta . However, since we measure the decay products from the Delta, this is likely to reduce back down to rank 2.

However, in the nucleon case, there are limits on the maximum M involved in the exchange, and it seems that these would have to be relaxed in the Delta case. The PWA consensus is that:

1. We are currently trying to understand the simpler problem of photoproduction off a nucleon. This has never been done off a delta.
2. This is a tractable problem, but presently we do not how and we would be uncomfortable defending a proposal on this without accumulated experience.
3. This is likely to be very important physics at some point, especially when we want to say we have solved the baryon resonance problems about which we were talking yesterday. However, the problem is complicated enough that we are uncomfortable with it without additional study.

Assuming that rho pi is a good production mechanism for the 1.6 GeV exotic, which other decay modes should be searched?

Other Stage-1 Physics

S/P interference under the phi. This sheds light on the nature of the ao(980) and the fo(980) and is an important tool for CP/CPT physics. This implies sensitivity to the two body states:

Hybrid Spectroscopy below 1.9 GeV.

s-sbar Spectroscopy below 1.9 GeV.