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The Crystal Ball at MAMI A program of precision photonuclear measurements is now possible using the combination of the 4p Crystal Ball (CB) multiphoton and neutron spectrometer and the TAPS detector, using tagged, polarized photons and polarized p and d targets. This facility will allow us to measure meson photoproduction on a neutron target, GDH on the neutron, a variety of double polarization g® N® measurements for both proton and neutron targets, threshold strangeness photoproduction, and the first measurement of the magnetic dipole moment of the charged and neutral D(1232) and S11(1535) resonances. In consideration of this, the SLAC Crystal Ball has been moved from the AGS facility at BNL to MAMI in Mainz, Germany. The director of MAMI has made it clear that this facility will play an important role in MAMI's future (see the attached letter). A new collaboration, the Crystal Ball with TAPS at MAMI (CBTM) Collaboration has been formed by about sixty physicists who share our high expectations for the physics opportunities that will become available with CBTM. Currently, the 855-MeV electron beam (MAMI B) can produce tagged photons up to 800 MeV in 2-MeV bins, with significantly better energy resolution possible using the focal-plane microscope. The upgrade to 1500-MeV electrons (MAMI C), which will provide tagged photons up to 1400 MeV, will be completed by early 2005. As at JLab, linearly polarized photon beams are produced from coherent bremsstrahlung, and circularly polarized beams from longitudinally polarized electrons. A frozen-spin target for 4p particle detection (allowing free passage of particles in all directions) has also been developed and fits within the entrance tunnel of the Crystal Ball to provide longitudinally polarized nucleons. One section of the TAPS detector will become the forward detection array of the CB, substantially improving the acceptance at forward angles. We have made plans for a three-phase, open-ended program. Phase I will be performed under the present MAMI-B conditions, a moderate upgrade of the Crystal Ball, and standard LH2 and LD2 targets, with some possibility of polarized targets. In Phase II, we will make use of the MAMI-C upgrade to 1.5 GeV. Here again we will start with LH2 and LD2 targets and use polarized photons for the first time at MAMI-C energies. In Phase III, we will concentrate on meson photoproduction reactions with polarized beams and polarized p and "n" targets. By the time of Phase III, we hope to have a working polarized-3He target to use as a neutron source. We have been assured by the management at MAMI that we will receive on the order of 2000 hours of beam time per year for the next several years; current plans for Phase I include about 2000-2500 hours of beam in 2004. The tagged-photon flux at MAMI of Ng ~ 105 s-1 MeV-1 is 10-100 times larger than at other facilities. In addition, a frozen-spin polarized target has been developed in collaboration with groups at Bonn and Bochum [Bra99]. The combination of high photon flux, both linearly and circularly polarized beams, a polarized target, and a 4p detector with TAPS as an end cap presents an excellent opportunity for experiments in the first and second baryon resonance region of very high interest and quality. Experiments can be done that are either not feasible or would take too long at other laboratories. While only one "proposal" for a specific experiment has been formally approved by the MAMI PAC for our first running period this Fall, a full program has been presented to the PAC and submitted by the laboratory to the German funding agencies, who have responded with significant funding. We highlight below a few specific topics of the program that are of interest to us. Additional data relevant to baryon spectroscopy and meson photoproduction and decay will, of course, be obtained in parallel. Full details will be developed in future PAC proposals for individual experiments to the MAMI Program Advisory Committee. |
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