Scientists Report First Experimental Results from Jefferson Lab

Certain to be one of the most important new experimental nuclear physics facilities in the world, the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, produces high energy electron beams that collide with nuclei in order to study the boundary between the physics of the nucleus and the physics of protons and neutrons (composed of quarks held together by particles known as gluons). At a Friday afternoon session at the APS/AAPT Spring Meeting in Washington, DC, researchers presented some of the first experimental results obtained from Jefferson Laboratory by four large team-conducted projects. The experiments, selected by a panel of internationally prominent physicists, focused on achieving a definitive quark-based understanding of the atomic nucleus.

The main machine at the $600 million laboratory is the Continuous Electron Beam Accelerator Facility (CEBAF), an underground tunnel almost a mile in circumference which accelerates continuous streams of electrons to energies of 4 GeV. A maximum energy of 8 GeV is planned for the future. The electrons are then diverted to one of three experimental halls where they collide with fixed targets containing nuclei, while house-sized arrays of electronic data-gathering equipment track, measure and record what occurs. Director Hermann A. Grunder describes it as a research tool designed "not so much to smash atoms as to dissect them." Because the beams are continuous rather than intermittent, experimenters can avoid unwanted, confusing signals ("background noise") in the electronic evidence they are studying. "Instead, they can dissect nuclei with a scalpel-like precision unattainable in previous 'atom smashers,'" said Grunder.

Exploring how gamma rays break up deuterons, Haiyan Gao of Argonne National Laboratory presented measurements showing that the quark substructure inside the deuteron must be taken into account to properly understand the breakup process, known as "photodisintegration." The deuteron is a simple nucleus consisting of just a proton and a neutron, each made up of three quarks, which enables it to be probed for useful data without too much complexity. The probing was done with photons extracted from the electron beam. When a photon disintegrates a deuteron, all of the deuteron's quarks are temporarily forced into a cluster of pure quark matter. Deuterons were thus probed in 1968 at SLAC to provide proof that quarks exist. According to Gao, her experiment reproduced the old 1968 picture, enlarged it and clarified its details.

Describing experiments in which electrons collide with hydrogen, deuterium, and carbon nuclei, Keith Baker of Jefferson Lab and Hampton University reported on the production of kaons, two-quark states that contain a strange quark. The experiments observed the transfer of electron energy to clusters of three ordinary quarks: two "up" and one "down." As the deposited energy causes one of these ordinary quarks to be stretched away from its partners, two exotic new quarks are formed: a "strange" quark and an "antistrange" quark. Together, the antistrange quark and the stretched "up" quark form an easily detectable kaon, Baker reported. And when detected in the precise experimental environment of Jefferson Lab, kaons yield vital new data about how quarks are made out of energy and how antimatter is produced.

Finally, Rolf Ent of Jefferson Lab described how electron collisions with nuclei are ejecting protons at a greater rate than anticipated by theory, based on the recently completed baseline-establishing experiment on photon propogation in nuclei. This information is crucial to future experiments to test and validate quantum chromodynamics at high energies, where nuclei often behave differently than they do at lower energies. A team of scientists from Caltech conducted an inclusive electron scattering experiment from nuclei which also helped prepare a baseline for interpreting deviations from conventional expectations that QCD predicts will appear at high energy.