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will be the final hall to begin physics operations (scheduled
for late 1996). The major apparatus in Hall B is a large-acceptance particle
detector (the CEBAF Large Acceptance Spectrometer or CLAS) based on a toroidal
magnetic field produced by six superconducting magnetic coils. The six sectors
between the coils are instrumented with drift chambers, Cerenkov counters,
scintillation hodoscopes, and electromagnetic calorimeters which identify and
determine the momentum of several, simultaneously-emitted, charged particles.
Additionally, the scintillation counters together with the calorimeter elements
will allow an approximate determination of the energy of neutral particles. The
continuous nature of the CEBAF beam is critical to the functioning of such a
multi-particle coincident detector. Hall B also includes a bremsstrahlung
photon tagging facility so that CLAS can investigate both real and virtual
photon processes.
A major research program for the CLAS is the investigation of the quark-gluon
structure of the nucleon, especially the detailed study of its spectrum of
excited states. The spectrum of this system contains vital information on the
nature of its constituents and the forces between them. It is not understood
why the naive constituent quark model is so successful in explaining the
particle spectrum discovered so far. CLAS will conduct sensitive searches for
the complete pattern of predicted, but hitherto unobserved states. This is
likely to reveal the limitations of the naive constituent quark model. The CLAS
spectrometer will also be used in a variety of other investigations requiring
data on multi-particle final states, including short-range correlations between
nucleons in nuclei, the importance of three-body forces in nuclei, and
modification of the nucleon's properties in the nuclear medium.