Evidence for the onset of quark effects in a nuclear reaction has been
observed for the first time. When a particle strikes a nucleus at low
energies, one can effectively describe the resulting behavior of the
nucleus in terms of its constituent nucleons (neutrons and protons)
and the mesons which hold them together. At low energies, one does not
have to worry about the fact that each nucleon is itself made of three
quarks held together by gluons.
When a particle strikes a nucleus at high energies, however, it penetrates
the nucleus so deeply that this "effective theory" breaks
down, and one must describe the nuclear action in terms of only quarks
and gluons. There is a middle ground, alas, where neither descriptive
picture can do the job completely.
Just as urbanologists strive to locate where a city truly ends and
its suburbs begin, physicists wish to find the boundary at which nucleon-based
descriptions give way to quark-based ones. Towards this end, researchers
study the behavior of the deuteron, the simplest nucleus, made of a
proton and a neutron bound together.
In experiments at Jefferson Lab in Virginia, a multi-institutional
collaboration (Elaine Schulte, Argonne National Lab, 630-252-4032, Schulte@mep.phy.anl.gov)
fired a high-energy electron beam at a copper target, which decelerated
the electrons, creating high-energy photons as a result. In a process
known as "photodisintegration," the photons impinged upon
a deuterium target, and broke apart deuterons into their constituent
protons and neutrons.
The researchers then studied the properties of protons emitted at various
angles from the collision. When the emitted proton has at least 1 GeV/c
of momentum perpendicular (transverse) to the incoming beam, the data
were best described by quark-counting rules, which take into account
the behavior of individual quarks.
The transverse momentum translates to an interaction with the nucleus
at a distance scale of 0.1 fermi (10-16 m), about a tenth
of the width of a proton. In this situation, an individual quark, rather
than the entire nucleon, absorbs the momentum of the collision. This
was surprising, since the 0.1-fermi distance scale is larger than many
current theoretical expectations for the onset of quark-counting-rule
Schulte et al., Physical Review Letters, 3 September