Outlook and Open Questions

The study of photo-and electro-production of mesons in the region of nucleon resonances provides us with rich and much needed information about the transition amplitudes from the ground state nucleon to the and the orbital and radial excitations of the 3-quark multiplets in . Their detailed study within the program at CEBAF aims at a better understanding of the quark-gluon structure of light quark baryons in the non-perturbative regime of QCD, and the transition to the perturbative regime. Due to the high quality and completeness of the expected data, stringent tests of QCD-related models can be provided. These models include QCD sum rules, dynamical quark models, as well as QCD lattice calculations. Eighteen experiments have already been approved to measure single , , and production throughout the resonance region, both in photoproduction and electroproduction. Several experiments will measure polarization observables, including beam and target polarization as well as recoil proton polarization. The bulk of experiments will make use of the CEBAF Large Acceptance Spectrometer (CLAS) which allows measurement of complete angular distributions and of several reaction channels, simultanously. These data will provide the information required for a detailed partial wave analysis in a coupled channel approach, covering the entire resonance region.

The non-perturbative internal structure of light baryons like the proton and the are of central interest in electromagnetic physics. The ``deformation" of these states due to the tensor force in one gluon exchange is a key piece of information which can be obtained with the new facilities at CEBAF, LEGS, and Bates. The non-S-state components of the wavefunctions of these states leads to non-zero contributions in the E2 multipole of the transition. In quark models the ratio of E2 to M1 is zero or very small at low , while the perturbative QCD limit is unity as infinity. The ratio is a direct probe of details of sub-nucleonic dynamics. The evolution of this ratio is therefore important to determine. Information on the poorly known `Roper' resonance will help establish the QCD structure of this state, which in the quark model is assumed to be a N=2 radial excitation, while some recent models favor it as the first nucleon state with a large gluonic component (hybrid). Measurement of the dependence of the transition amplitudes will help to distinguish between these models.

The extraction of photon couplings for baryon resonances requires a precise determination of the helicity structure of their amplitudes. Even at this would require knowledge of 7 polarization observables. In the absence of sufficient measurements, traditional analyses have essentially locked photoproduction to scattering. Although this has accounted for the overall trends of existing data, many conflicts persist. New polarized beams and targets provide the capabilities needed for direct measurements of the helicity structure. For example, experiments are planned for LEGS that will scan through different polarization orientations to measure 6 of the observables for both proton and neutron targets.

In hadron spectroscopy, the search for new and exotic states is motivated by our drive to identify the underlying degrees of freedom in the baryons and mesons. Just as nuclear structure studies led to our understanding of the dynamics of nucleonic many-body systems, the next generation of experiments at CEBAF will inform us of the dynamics of quark and gluon many-body systems. A well-known example is the spectrum of baryons predicted in the SU(6)xO(3) representation. Our current knowledge of the the light quark baryon spectrum comes almost entirely from -channel pion scattering from nucleon targets, and many of the states predicted by the quark model have not been found. Whether this results from the limitations of past experiments or from new underlying dynamics which supresses these states is a high priority question today. In fact, the established states correlate very well with a model based on diquarks. Perhaps di-quark structures reduce the symmetry system, thereby removing certain states. On the other hand perhaps the unseen baryonic states do not couple strongly to channels, and hence went undetected in the work done in the past.

At CEBAF the attack on the ``missing" baryons issue rests first on using photons rather than pions to couple to such states, and second on examining non- final states. At least seven electro-and photo-production experiments on the books will detect final states such as , and . These will be high statistics measurements with good control of systematics. Excellent statistics and control are required for such discovery searches, as well as the `microscopy' program above. This ambitious program is an important focus of the CEBAF physics program in the next five year period.

Strangeness production in hadron structure studies adds another flavor to electromagnetic physics. Unlike the case of single pion electroproduction, for example, where the is the single dominant resonance at low , very little is known about the resonance structure of single kaon electroproduction. Essentially no L/T separations have been done, for example. More than one CEBAF experiment will address this. Radiative decays of the excited hyperons are a probe of quark structure; the structure of the , which may be a molecular KN state, will be investigated at CEBAF. Another pair of purported strange molecular states, the , will be probed in electroproduction to examine the dependence of its production and hence its spatial structure.

In meson spectroscopy, CEBAF may hope to discover evidence for `exotics' with quantum numbers forbidden by the simple quark model. An example is the state near 1900 MeV predicted to decay to . Such states are allowed with explicit excitation of the gluonic flux tube which links the quarks. The real photon beam at CEBAF will serve as a clean source of vector mesons and perhaps also of such exotic species. Experiments to persue this type of spectroscopy are under development at CEBAF. In order to have sufficient phase space for production of such exotics, which have predicted masses above about 2.0 GeV/c, the practical lower limit on the beam energy is about 6 GeV. Hence these studies fit best into the plans for eventually upgrading CEBAF to higher energies in the 8 to 10 GeV range [St94].

Spectroscopy with photoproduced mesons may offer another avenue of development in the next five-year period. A workshop was held at Indiana University in the spring of 1994 [IU94] to discuss the physics possiblilities offered by a ``flying phi" factory. The is the prototypical state, and it is produced copiously in reactions at high energy. Measuring radiative decays of the to the would test the internal structure of the latter state. The decays to -odd correlated pairs, and is therefore a natural laboratory for precision tests of and violation. Such a ``flying-'' factory has several advantages over planned machines. These include, for example, higher lab-energy decay products which facilitates background rejection, and the use of interference to enhance the symmetry violation signals.