In electron scattering we may
identify 3 distinct kinematical regions corresponding
to different distance scales.
At high energies and small distances the interaction
involves elementary quark and
gluon fields, acting as quasifree particles.
The interaction is described
by perturbative QCD. At low energies and large distances,
quarks and gluons appear in
'condensed' form as nucleons and mesons, and the reaction
is described by hadron theory.
At intermediate distances, which we are concerned with,
quarks and gluons are relevant,
however confinement plays a governing role, and quarks
and gluons appear as constituent
quarks and constituent glue, as for example in the flux
tube model. This picture, although
quite successful in describing many aspects of hadron
spectroscopy, is a model whose
relationship to QCD remains unclear. One of the goals of
the N* program is to provide
accurate data that can be confronted with model calculations
and show where this picture
breaks down in non-trivial ways leading to improved models
and to a better understanding
of the nucleon structure in terms of its fundamental constituents. The
goal of the N* program at CEBAF is to probe the internal structure of
light quark baryons. This will
be accomplished in various way:
(1) by studying the resonance transition formfactors in a large Q2 range and for a large number of states. This will allow detailed tests of baryon structure models,
(2)
by searching for the so-called "missing resonances", states
which are predicted in the symmetric
quark model but have not been seen so far, and
(3) by searching for gluonic excitations of the nucleon ("hybrid baryons").
The constituent quark model (CQM)
in its various implementations (non-relativistic, relativized) is a useful
guide for the experimentalist. It provides physical insight
and is aimed at a global description
of both the mass spectrum as well as the structure
of hadrons within a common framework.
The model predicts a large number of resonant
light quark (u,d,s) baryon states
within the symmetry group SU(6) x O(3). The states
fall into supermultiplets with
fixed orbital angular momentum and energy excitation level.
The mass degeneracy within one
supermultiplet is broken by the color magnetic hyperfine
coupling between the quark spins.
Transitions within the ground state N\Delta, and from
the ground state into states
with excitation levels 1 hv and 2 hv are of special interest
since more accurate measurements
can be made for the lower mass states, and small, but
important effects can be studied.