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

THE SCIENCE • PROTONS AND NEUTRONS: STRUCTURE AND INTERACTIONS
ized data from CEBAF, shown in Figure 2.10, appear to be
consistent with expectations from either the quark-gluon or
the meson-nucleon view. To understand the range of applicability
of the two pictures, it is essential to extend these and
related measurements in deuterium and other light nuclei to
the highest possible values of momentum transfer, where
sensitivity to the quarks alone would be enhanced. Such a
program can be carried out by upgrading the CEBAF accelerator
to 12 GeV.
Another avenue for investigating the role of quarks in
nuclei is by using high-energy gamma rays to break up
the deuteron into a proton and a neutron. In general, the
momentum transferred to the two nucleons can be substantially
higher than in the case of electron scattering. It appears
that high-energy two-nucleon breakup of deuterium is, in
fact, consistent with the quark-gluon picture, whereas available
meson-nucleon models fail to explain the data. Recent
polarization data show very similar behavior, which would
be completely unexpected in a meson-nucleon picture.
Even the best available model of the NN force cannot
accurately explain nuclear binding. To reproduce the binding
energies of the simplest light nuclei, it is necessary to add
three-body forces to the pairwise interactions determined
from nucleon-nucleon scattering (see “Theoretical Advances,”
pages 30–31). Unless such forces are considered, the binding
energies of light nuclei are too small, and the binding of
nuclear matter, relevant for understanding neutron stars, is
too large. Precise new polarization data from the Indiana
University Cyclotron Facility (IUCF) constrain the calculations
of the spin-dependent part of the three-body force.
Theoretical Advances
In recent years, theoretical investigations of the internal
structure of the nucleon, and hadrons in general, have
focused mainly on achieving close ties to the fundamental
theory of strong interactions, QCD. At present, three
approaches are promising. The first is numerical simulation
of lattice field theory, the only way known to directly solve
“strong” QCD with controlled errors. A second approach
focuses on low energies and exploits the spontaneous breaking
of chiral symmetry that is an important consequence of
strong QCD. This is the effective field theory approach (see
above) that has been used with great success in establishing
the Standard Model of electroweak and strong interactions.
f d A
10 – 4 JLab Hall A
SLAC E101
10 – 5
10 – 6
10 – 7
10 – 8
10 – 9
0.04
0.03
0.02
0.01
Reduced deuteron form factor
f d (Q 2 ) = F d (Q 2 )/F N
2 (Q
2 /4)
0.00
0.5 2 4 6
Q 2 [(GeV/c) 2 ]
pQCD
Figure 2.10. Electron-deuteron scattering results. Measurements
of unpolarized electron-deuteron scattering cross sections from
SLAC and Jefferson Lab, characterized by the structure function
A(Q 2 ), are consistent with meson-nucleon–based theories of
nuclei (upper figure), while at higher Q 2 , they also become consistent
with the behavior predicted by quark-gluon–based theories
(lower figure).
The third approach is perturbative QCD, a powerful tool to
extract the structure of the nucleon from high-energy scattering
processes.
Lattice QCD. Although the idea of lattice regularization
was introduced shortly after the advent of QCD, only
recently have the algorithmic, analytical, and computational
tools been developed to the point that lattice QCD calculations
can have a major impact. In particular, practical methods
have been found for incorporating an exact form of
chiral symmetry on a lattice, and all the tools are now at
25