(Briscoe, Strakovsky)

We plan to exploit the unique photon- and neutron-detection properties of the Crystal Ball and TAPS at MAMI by measuring the simple photoproduction reactions gn ® p^on, gn ® hn, gn ® h'n, and gn ® wn from threshold to 1.5 GeV, and in this same energy region the cascade decays gn ® p^op^on and gn ® p^ohn. We will further explore the details of the electromagnetic properties of resonances by searching for the radiative-decay chains that result in a p^og or hg final state such as N* ® gD or N* ® gS₁₁(1535). These measurements will first be carried out using an LD₂ target to provide the target neutrons in measurements of total and differential cross sections as well as beam asymmetry measurements. Then we will use a polarized deuterium target to make double-polarization measurements. These measurements are completely complementary to the charged final-state measurements at JLab; they will further the investigation of low-lying resonances and will contribute to the search for the "missing" resonances.

In order to extract reliable N* resonance parameters from the meson-photoproduction data which are now coming out of JLab, GRAAL, ELSA, and MAMI, partial-wave analyses must be extended to include additional channels and reactions. It has become increasingly evident that single-channel pN analyses alone do not provide the necessary constraints needed for a full and unambiguous determination of resonance properties. Complete measurements with all combinations of meson and nucleon isospin are needed to determine and test the amplitude analysis process.

One of the goals of the N* program at JLab is to search for the "missing resonances" - unobserved states predicted by constituent quark models [Lee97]. Approved Hall-B experiments propose to measure channels other than single-pion photoproduction, under the assumption that resonances that couple strongly to the pN final state would have already been seen in pN elastic scattering. Experiments are in progress that measure multipion final states as well as h, h¢ , r, w, and f production. However, these states could also be unobserved because they are only weakly coupled to the measured charged pN channels. That is, like the neutral Roper resonance, they can be strongly coupled to the neutral p^on final state and not be seen in any of the charged elastic pion-scattering channels or in the photoproduction of the charged pN final state. The CLAS in Hall B is not suitable for measurements that have all neutrals or even a multiplicity of neutral products.

Although at JLab little attention is being paid to the detailed study of low-lying resonances despite the considerable interest evident at other labs [Kru99], some measurements are being made for that purpose. Here again the CLAS trigger requires the presence of at least one charged particle in the final state because the solid angle for photons is limited and the detection efficiency for low-energy neutrons is very small. Recent neutral-channel measurements at ELSA and MAMI have yet to produce precise results for photoproduction of mesons from the neutron because of the lack of full 4p acceptance for photons and neutrons; only coherent and incoherent photoproduction cross sections from the deuteron have been presented to date [Kru99].

While a major justification for building the CLAS at JLab was the need to study meson-photoproduction reactions, the reality is that many channels of interest include final states which are either all neutral or eventually decay to multiple photons. While CLAS is well-suited for charged-particle final states, recent attempts to measure gp ®  2p^op [Phi02] at JLab have led to the conclusion that CLAS is not the device of choice for studies in which multiple photons (and/or neutrons) must be detected. The inability to measure all-neutral final states to any reasonable accuracy over more than a very limited angular range leaves significant gaps in the baryon-resonance program at JLab. For example, the radiative coupling of neutral resonances (e.g., the neutral Roper) cannot be studied thoroughly at JLab using CLAS. The Crystal Ball Program at MAMI thus can provide a necessary complement to the JLab N* program.

The Crystal Ball by itself is an "almost 4p" photon spectrometer. Including TAPS as an end cap to the Crystal Ball increases the coverage in the forward direction. While this already provides a unique opportunity to measure reactions in which one or more of the final-state particles decay into photons, the fact is that both the Crystal Ball and TAPS also have high detection efficiencies for neutrons [Sta01]. This, coupled with the excellent energy resolution of the MAMI tagged-photon beam, provides an opportunity to make precision measurements of the sparsely measured neutral photoproduction channel gn ® p^on, providing high-quality input to the new SAID and MAID analyses. This channel has never been measured by detecting both photons and neutron (except for one exploratory measurement at Daresbury [Cli74]) using a tagged-photon beam.

While only the position information of the neutron is measured in the Crystal Ball, in the forward direction a measure of neutron energy is given by TAPS. This information provides a way of extracting a measure of the recoil of the spectator proton. Thus, it is expected that the measurements made at MAMI will not only have lower background, but also a better handle on the problem of using a neutron bound in the deuteron as a target. In all cases, the neutron-detection efficiency is known to be about 35% [Sta01]. We are currently discussing the problem of corrections for the fact that the neutron is bound in a nucleus with our theoretical collaborators at GW and Mainz.

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