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

THE SCIENCE • PROTONS AND NEUTRONS: STRUCTURE AND INTERACTIONS
cally use beams of leptons (i.e., electrons, muons, or neutrinos)
or photons over a broad range of energies to probe
nucleon structure over many distance scales. Such experiments
also provide especially sensitive ways of distinguishing
the different “flavors” of quark, notably the up and
down quarks that help to give protons and neutrons their
separate identities. At low momentum transfers, static
properties of the proton and neutron, such as their shape,
size, and polarizability, are determined and compared with
QCD-inspired models and lattice calculations. At high
momentum transfers, the spatial, spin, and flavor structure
of the proton and neutron are probed by scattering from the
elementary quark and gluon constituents.
A view to the future. Over the past decade, a new generation
of experimental capabilities has been put in place. Highduty-factor
beams of electrons, polarized beams and targets,
and revolutionary new detectors have become available, producing
unexpected, and not yet fully understood, results.
With these tools, we have gained new insights into proton
and neutron structure. For the future, important new devices
and capabilities are under development, including a pure,
solid hydrogen or deuterium target (for experiments at the
Laser Electron Gamma Source facility at Brookhaven) and
intense polarized photon beams (the High Intensity Gamma
Source facility at Duke). At MIT-Bates, the new Bates Large
Acceptance Spectrometer Toroid, coupled with the intense,
highly polarized electron beams and pure polarized-gas targets
internal to the South Hall Ring, will provide high-precision
data on nucleon structure at long distance scales. Each of
these devices will add critical capabilities in the near future.
In the longer term, the proposed 12-GeV upgrade of the
CEBAF accelerator at Jefferson Lab will greatly extend the
scientific reach of this facility and open new opportunities for
the program. The hybrid mesons described above, for example,
are expected to be produced by photon beams that will
be available at an upgraded Jefferson Lab. A complete understanding
of the fundamental structure of matter will require
continued experiments over a broad range of energies and the
continued development of new devices and capabilities.
The Building Blocks of Matter:
The Structure of Protons and Neutrons
Detailed investigations of the structure of the proton and
the neutron are essential for understanding how these basic
building blocks of nuclear physics are constructed from the
quarks and gluons of QCD. Remarkable new data are now
becoming available that shed a revealing light on hadron
structure, and yet much remains to be learned.
Elastic form factors of nucleons. The electromagnetic
form factors of a nucleon provide a picture of the distributions
of its charges and currents, due almost entirely to up
and down quarks. High-energy hard-scattering data have
shown that, in addition to these quarks, there are also antiquarks
inside the nucleon, some of which are probably
responsible for the nucleon’s pion cloud, though the precise
details have been unclear. Recent precision measurements
of the electric and magnetic polarizabilities of the proton
have now revealed the role of pions in nucleon structure.
Data with comparable precision are still needed for the neutron,
and available results are in serious disagreement with
theory. Existing and planned low-energy facilities will be
able to make extremely precise measurements for the neutron
polarizabilities in the near future.
An experiment at Jefferson Lab has found that the charge
and current distributions in the proton are not the same. By
measuring the polarization of the outgoing proton in elastic
electron scattering from a proton target, experimenters
determined the ratio of the electric and magnetic form factors,
G E and G M , respectively, with small systematic uncertainties.
The data, depicted in Figure 2.1, show that G E and
G M behave differently as the resolution of the “microscope”
(the momentum transfer Q 2 ) increases.
Comparable precision measurements for the neutron are
much harder, not least because a practical pure neutron target
cannot be produced. The total charge of the neutron is
zero, as the net contributions from its positively (mostly u)
and negatively (mostly d) charged quarks counterbalance
each other. However, within the neutron, there is a distribution
of electric charge that has been revealed in recent
years by a number of experiments using 2 H and 3 He targets,
often involving polarization techniques. Results from
experiments at higher resolution are expected shortly. New
precision data on the magnetic structure of the neutron have
also been obtained.
The high-energy hard-scattering data have shown that
strange quarks and antiquarks (ss - ) also play a role inside the
nucleon, in particular by contributing to the spin of the proton
and neutron. The question of how these strange particles
cooperate in constructing the nucleon is currently being
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