OPPORTUNITIES IN NUCLEAR SCIENCE A Long-Range Plan for ...

THE SCIENCE • PROTONS AND NEUTRONS: STRUCTURE AND INTERACTIONS
pared with pion, kaon, or proton beams. To date, most
meson spectroscopy has been done with these latter probes,
so it is not surprising that the experimental evidence for flux
tube excitation is tentative at best.
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Elastic π+e
Previous p(e, e'π + )n
JLab
High-flux photon beams of sufficient quality and energy
will be become available at Jefferson Lab when the facility is
upgraded to 12 GeV. This project will accumulate statistics
during its first year that will exceed existing photoproduction
data by at least a factor of ten thousand and data with pions
by at least a factor of a hundred. With the planned detector,
high statistics, and linearly polarized photons, it will be possible
to map out the spectrum of these gluonic excitations.
Theoretical input, particularly from the lattice, will also
be needed to compare with the observed spectrum of states
and eventually their decay patterns. Recent improvements
in calculational techniques, coupled with the current lattice
initiatives to build a new generation of computers, will
make definitive calculations possible in the not-so-distant
future. When the spectrum and decay modes of gluonic
excitations have been mapped out experimentally, a giant
step will have been taken toward understanding one of
nature’s most puzzling phenomena, quark confinement.
Yet another important issue in the physics of confinement
is understanding the transition of the behavior of
QCD from long distance scales (low Q 2 , where confinement
dominates and the interaction is very strong) to short
distance scales (high Q 2 , where the quarks act as if they were
free). The pion is one of the simplest QCD systems available
for study, and the measurement of its elastic form factor
is the best hope for seeing this transition experimentally.
Figure 2.8 shows how the proposed CEBAF 12-GeV
Upgrade project can explore this transition.
The QCD Basis for the NN Interaction
At present, the best quantitative description of the force
between two nucleons remains the phenomenological
model of meson exchange. The long-range part of this NN
force is mediated by pions, the lightest mesons. The shortrange
part is less well understood. While lattice QCD computational
techniques are likely to provide detailed
predictions of the properties of single nucleons, numerical
solutions of QCD for systems of more than one nucleon are
still tremendously challenging. However, in the next few
Q 2 F π
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0
0 2
4 6
Q 2 [(GeV/c) 2 ]
pQCD
Figure 2.8. Projections for an upgraded CEBAF. The 12-GeV
Upgrade will allow measurements of the pion elastic form factor
through the expected transition region from confinement-dominated
dynamics (at modest Q 2 ) to perturbative-dominated dynamics (at
high Q 2 ). The colored symbols depict the expected precision at
values of Q 2 ≤ 6. The curves represent model predictions of this
transition, while the pQCD result (lower right) is the predicted value
at high Q 2 .
years, lattice QCD will be able to provide qualitative
insights into the interaction between two very massive
hadrons (containing quarks more massive than u and d),
which is a more soluble problem and involves the same
mechanisms of quark interchange and gluon exchange that
occur in the NN force.
Developing a deeper understanding of the origins of
the effective NN force in terms of the fundamental constituents
of QCD has recently become a realistic goal for
nuclear physics through the use of effective field theories.
These theories exploit the symmetries of QCD and enable
its confrontation with low-energy observables. Combined
with the computational techniques of lattice QCD, these
methods have the potential to provide a powerful quantitative
tool to connect QCD directly to the low-energy properties
of nuclei.
Understanding the NN interaction is vital not only for
gaining a clear picture of nuclear structure under normal
conditions, but also for making reliable predictions for
more extreme processes, such as those that take place in
supernovae or when a hot, dense quark-gluon plasma condenses
into nucleons and mesons. Effective field theories
are now very successful in describing processes involving
two nucleons at low energy, for example, n-p capture and
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