Onset of Quark Effects

Evidence of the onset of quark effects in a nuclear reaction has been observed for the first time [September 2001] (E. Schulte, et al., Phys. Rev. Lett. 87, 102302-1 [2001]). When a particle strikes a nucleus at low energies, the resulting behavior of the nucleus in terms of its constituent nucleons (neutrons and protons) and the mesons that hold them together can be effectively described. The fact that each nucleon is itself made of three quarks held together by gluons is not critical at low energies. When a particle strikes a nucleus at high energies, however, it penetrates the nucleus so deeply that this "effective theory" breaks down, and the nuclear action must be described in terms of only quarks and gluons. There is a middle ground, alas, where neither descriptive picture is adequate. 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, which is made of a single proton and a neutron.

In the E96-003 experiment at the Thomas Jefferson National Accelerator Laboratory (JLab) in Virginia, a high-energy electron beam was fired at a copper target that decelerated the electrons, creating high-energy photons. The photons impinged upon a deuterium target and broke apart deuterons into their constituent protons and neutrons, a process known as "photodisintegration." The researchers then studied the properties of protons emitted at various angles from the collision.

When the emitted proton had 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 result was surprising, since the 0.1-fermi distance scale is larger than many current theoretical expectations for the onset of quark-counting-rule behavior.

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