Physics Possibilities for CEBAF at Higher Energies

In view of the technical feasibility of a CEBAF upgrade into the 10 GeV range, a workshop was held in April 1994 to assess the physics case for running CEBAF at higher energies [St94]. A strong set of goals was established for electromagnetic physics with beams of up to 10 GeV. An overview of the areas of physics that would be addressed at 8 GeV is shown in Table . Several prominent examples could be discussed.

Studies of scaling in the valence region (large ) can be used to explore the duality of the quark and hadron pictures of scattering processes. In particular, it offers the exciting possibility of studying scaling when only a few distinct hadronic states combine to produce a partonic result. Experiments in the valence region will complement those at SLAC or HERMES. They would lead to precise structure function data with which to examine the approach to scaling.

Meson spectroscopy provides some of the most exciting new physics to be pursued at CEBAF with beams of 6 GeV and above. The only hadronic systems in which non degrees of freedom can be identified unambiguously are -exotic mesons, and such things must be present at some level in QCD. Exotic quantum numbers arise naturally in flux tube excitations of vector mesons, and so a photon beam is an obvious way to create such objects. Gluonic excitations cost energy, however, and the mass of exotic mesons is predicted to be in excess of 2 GeV/c or so. They would be produced exclusively in reactions like , and the threshold beam energy for this process is greater than 4 GeV. In order to have a large enough phase space for a significant cross section, a practical lower limit on the usable beam energy is around 6 GeV. Furthermore, these mesons may well have very complicated decays, such as , so large solid angle ``hermetic'' detectors with good charged and neutral capability are needed.

A particularly useful byproduct of such a program to study these gluonic excitations would be the spectroscopy of conventional mesons. These ``strangeonia'' represent an important link between the low mass, light quark mesons, and the charmonium system. However, our knowledge of these states are poor because their production is OZI-suppressed in reactions, and data with beams is not (and will never be) extensive enough. Photons, on the other hand, are fine for producing pairs, and are in fact the ideal way to study such objects. Again, however, the interesting states would have masses which exceed 2 GeV/c.

Color transparency is a novel QCD effect which has been explored in hadronic interactions. An 8 GeV beam at CEBAF would enable experiments to sweep the kinematic variables and through the relevant ranges of nuclear size, formation length, and coherence length for a full exploration of this phenomenon.

In the scenario presented to the community, cryomodule production for this upgrade would begin in FY 1999, with installation in FY 2000 and 2001. This upgrade would not interrupt production running of the machine at 4 GeV. Evolutionary detector upgrades would in this phase of the project come from the ongoing equipment capital budget. Many of the new physics goals could be addressed with essentially the detectors already on the floor. Present estimates of the cost of this first phase upgrade total $30M, or about 5%of the total cost of the present CEBAF facility. A second phase of the upgrade would then address major detector changes, new spectrometers, or beamlines; such an upgrade is not part of the proposal being put forward at this time.