Proton spin plays key role in smash hits.

Proon spin plays key role in smash hits

Like mass and electrical charge, spin is one of the fundamental properties used to characterize subatomic particles such as protons. High-precision measurements now reveal that a proton's spin seems to have a surprisingly strong effect on the outcome of violent collisions between protons.

"We find that there are very large spin effects in very violent proton-proton collisions, where two protons bounce off each other," says Alan D. Krisch of the University of Michigan in Ann Arbor. "Our data make it quite clear that there are large spin effects . . . where [most theoretical models] had said there should be no spin effects."

The results highlight important details of proton behavior that theoretical physicists cannot yet explain on the basis of quantum chromodynamics, the current theory describing proton structure and behavior. They also confirm earlier experiments that suggested similar spin effects (SN: 7/7/84, p.5).

"These are tough experiments to do," Krisch says. "With our new measurements, we have much smaller errors and many more data points."

At the simplest level, a proton can be pictured as a tiny ball spinning about an axis. Normally, the axes of a collection of spinning protons would point in random directions.

Krisch and his collaborators measured the results of firing a beam of high-energy protons at a special, stationary target in which virtually all the protons are polarized, or spinning in the same direction. "It's a marvelous target," Krisch says. "Its proton-spin polarization -- 96 percent for reasons no one yet understands -- was the highest level ever achieved in any high-intensity, particle accelerator experiment."

The research, performed at the Brookhaven National Laboratory in Upton, N.Y., showed that when protons having an energy of 24 billion electron-volts smash into a stationary target of polarized protons, about 50 percent more of the incoming protons are deflected to the left than to the right. Theoretical arguments predict that equal numbers should scatter to the left and right. Moreover, as the collision energy increases, any spin effects that exist should become increasingly negligible.

"Our new high-precision data make it difficult to assume that this disagreement between theory and experiment will disappear because the [result] is a statistical fluctuation," the researchers conclude in a paper submitted to PHYSICAL REVIEW LETTERS. "Perhaps one should now try to gain some new theoretical understanding of strong interactions [nuclear forces] that is consistent with this and other large and unexpected spin effects."

Indeed, Krisch argues that proton spin experiments point to serious flaws in current theory. Others disagree.

"Some of the claims . . . that these experiments violate QCD [quantum chromodynamics] are gross overstatements," says Francis E. Close of the University of Tennessee in Knoxville. "They show interesting phenomena, but this is the sort of dynamics that quantum chromodynamic theory isn't equipped to handle yet."

"QCD has many successes, which you can't ignore," adds Charles Y. Prescott of the Stanford (Calif.) Linear Accelerator Center. "The theory's inability to explain the Brookhaven results is a problem for QCD but not necessarily a failure."

The team plans to repeat its proton-scattering experiments at even higher collision energies when a powerful particle accelerator in the Soviet Union is completed in 1993. "We'll be doing the first experiment at the new accelerator," Krisch says. "We'll do exactly what we just did at Brookhaven, first at 400 billion electron-volts, then later at 3 trillion electron-volts." Those experiments should establish whether the unexplained spin effects disappear or persist at higher energies.