Probing a "Sea" of Quarks

Scientists who conducted the Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) Hall A Proton Parity Experiment, or HAPPEX, believe they may have at least a partial answer for an outstanding question in nuclear physics: Where does a proton's spin come from? Syracuse University professor of Physics Paul Souder, a primary HAPPEX architect and co-spokesman for the international HAPPEX science team, will present results of the first round of HAPPEX experiments during a poster session at the American Physical Society's Centennial Meeting in Atlanta on March 21 - 26, 1999.

"We were able to rule out the more exotic measurements of the contributions of the strange-quark content of the proton," says HAPPEX co-spokesman and Williams and Mary physics professor John Michael Finn. "We have plans for future measurements to look at other nuclei. The eventual goal is to map the strength, the flavor currents in the nucleus: how much up, down and strange quarks contribute to the nucleon's structure."

A proton's "spin" displays an intrinsic property known as angular momentum. The momentum is defined as having a certain magnitude, as well as a directional movement within space. Spin is responsible for the magnetic field a proton generates (a property that has led to the creation of technologies as Magnetic Resonance Imaging, or MRI). But deep inelastic scattering measurements indicate that the net spin is not carried solely by the proton's three basic "valence" quarks (the two lightest quarks, known as "up" and "down"). SO where is it? Could it be due to the strange quark sea?

To find out, the HAPPEX team made use of a subtle property of subatomic forces. Most forces in nature act the same way in our world as in a mirror world, where right-handedness is transformed to left-handedness. The subatomic force known as the weak interaction does not obey this symmetry principle, which is called parity. HAPPEX made use of the parity violation to probe the way in which different flavors of quarks assemble themselves into the protons and neutrons that comprise the atomic nucleus.

In particular, HAPPEX was sensitive to matter/antimatter pairs of strange quarks that spontaneously appear and disappear within the nucleus. These "sea" quarks exist in addition to the three valence quarks that are the basic components from which protons and neutrons are assembled.