Low energy confrontations with QCD through PT is an area in which a
great deal of experimental activity is planned. The attacks are proceeding
on many fronts and at many different facilities.
In the area of threshold pion production, work is in progress at several
laboratories to measure the p-wave multipoles in and
photoproduction and the L
amplitude in electroproduction. Recent
calculations of threshold double-
production have predicted an
enhancement due purely to pion-loop effects, and measurements of this
process are in the planning stages at several facilities. It is also
possible exploit the unitarity relations between the
N and
N
channels in order to deduce more accurate
N scattering lengths from
the phases of the threshold photo-production amplitudes.
New information on neutron polarizabilities will be forthcoming from quasi-
free Compton scattering experiments from deuterium above -threshold.
In addition, new photoabsorption measurements will test the (
) sum rules.
Spin-dependent Compton scattering from the nucleon is a subject of great
current interest and offers many opportunities for experimental progress in
the next few years. The Drell-Hearn-Gerasimov (DHG) sum rule (and its
evolution using virtual photons) will be studied in detail at many
facilities in the near future. In addition,
PT relates the energy
dependence of the spin-dependent part of the forward amplitude to the
nucleon spin-polarizability,
. This quantity is the spin-
dependent analog of
and can be determined via the
weighted integral of the spin dependent photoabsorption cross
section.
In PT, processes involving only pions are particularly interesting
because of the rapid convergence of their calculations. Two present
interesting challenges: the polarizability of pions and the Chiral
anomaly. There have been several extractions of the polarizability of the
and, although there is no disagreement with
PT predictions,
the quality of the data need to be improved to test this solid prediction
of
PT. Further experimental work is planned at several facilities.
The Chiral anomaly is a consequence of the breaking of Chiral-symmetry in
the QCD Lagrangian due to quantization, and leads to a finite amplitude
for the decay which is in fact directly
proportional to the number of colors in QCD, N
. (The traditional proof
that there are only 3 colors in QCD comes from the prediction for
decay, which is a direct consequence of the
axial anomaly and is also proportional to N
.) There has been only one
measurement of the
amplitude, using
-production via the Primakoff effect in the Coulomb field of a high-Z
nucleus, and this experiment is consistent with N
=4. New data on this
process will be forthcoming and a new experiment is planned to measure the
Chiral anomaly using the
reaction.
Clearly, there is a great deal of activity in this area. However, the
cross sections are rather small and experiments generally require large
blocks of running time. In many cases polarization observables are highly
desirable or essential (eg., the proton and neutron spin-polarizability,
, the neutron polarizabilities,
and
, p-wave
threshold pion multipoles, and extraction of the Chiral anomaly amplitude
for
). Good facilities are either in
place or planned for the near future that could vigorously pursue this
program. With adequate support for relatively modest additions (eg., the
new polarized target and detector system at LEGS, the high-flux DFELL
source, the laser-backscattering capability for Hall B at CEBAF, and
facilities at the new MIT/Bates South Hall Ring), a great deal of progress
should be achievable over the next decade.