Putting a new spin on the neutron.

Composed of quarks and gluons, the proton should have characteristics, such as spin, reflecting those of its constituents. But how much of a proton's spin (which is the source of this particle's magnetism) comes from the spins of its three quarks has remained a puzzle. Five years ago, measurements at the European Laboratory for Particle Physics (CERN) in Geneva, Switzeriand, suggested that only a small fraction of a proton's spin is carried by the quarks. Instead, the gluons, which bind the quarks together, apparently make a significant contribution to the total.

This surprising finding prodded a host of theorists to try to explain the result (SN: 4/8/89, p.215) and prompted two independentfollow-upexperiments, oneatcern and the other at the Stanford Linear Accelerator Center (SLAC). At CERN, researchers studied how elementary particles known as muons bounced off a target made up of deuterons (each comprising a proton and a neutron). The SLAC group tracked electrons scattered by a target made up of helium-3 nuclei (each composed of two protons and a neutron). In both cases, taking advantage of the close relationship between the proton and neutron, the groups determined the neutrorfs spin content as a check on the original proton findings. Now the results of the follow-up experiments are in, and the question of the origin of the proton's (and neutron's) spin is still unsettled.

In the Aug. 16 PHYSICAL REVIEW LETTERS, the SLAC team presents results suggesting that a neutron's quarks carry about 57 percent of the neutron's spin, with an error margin of 11 percent on either side of this value. On the other hand, the CERN group deduces from its results that a neutron's quarks carry only 6 percent of its spin, but the margin of error (25 percent) is much larger. The CERN determination supports the original proton-spin findings, whereas the SLAC result is more in line with theoretical expectations.

Because the two experiments were performed under quite different conditions, comparing the two sets of measurements directly has proved difficult. "There is a range of possible values that sort of spans what the two experiments say," says Timothy E. Chupp of the University of Michigan in Ann Arbor, who participated in the SLAC effort. What's needed is a second round of experiments to narrow that range and reduce the error, he says.

"We're making good measurements, and in a relatively short period of time on the scale of high-energy physics, we're converging on definitive answers," Chupp notes. "Even the current results are allowing theorists to focus theirefforts quite a lot."