PHYSICS ON A SHOESTRING: CREATIVE EXPERIMENT TESTS 40-YEAR- OLD THEORY

FOR IMMEDIATE RELEASE

SANTA CRUZ, CA--It seems that today's research in particle physics takes tens of years, hundreds of researchers, and millions--if not billions--of dollars. But that's not always true. Just ask the 11 researchers who spent about $10,000 on a clever experiment that lasted a few months--and supported a theory that physicists had not tested rigorously for 40 years.

Not only that, but the experiment needed no expensive allotment of time on a particle collider. It "parasitized" a larger operation by using particles that researchers normally throw away.

Physicists at the University of California, Santa Cruz, led the trend-bucking experiment, known as E-146, at the Stanford Linear Accelerator Center (SLAC) last year. E-146 measured an odd interaction among particles that happens over the extraordinarily large distance (for particle physics) of up to a millimeter.

Probing the interaction was not just an academic exercise. "This effect was predicted in the 1950s," says the E-146 spokesman, postdoctoral researcher Spencer Klein of UCSC's Santa Cruz Institute for Particle Physics. "Given the state of accelerators and particle physics at that time, it was purely theoretical. But now physics has grown up, and it's relevant in a number of areas."

For instance, physicists must account for the effect when they design detectors to measure energies in new particle colliders, study showers of particles caused by cosmic rays slamming into the atmosphere, or theorize about reactions inside stars and supernovas. Similar effects occur when a subatomic particle travels through an atomic nucleus.

The phenomenon is called the "LPM effect," after Landau, Pomeranchuk, and Migdal--the Russian theorists who proposed it in 1953. It occurs when an electron zips through matter, reacts with a nucleus, and loses energy in the form of a gamma ray. The electron is so fleeting that the interaction transfers a vanishingly small amount of momentum to some of the particles. As a result, the uncertainty about exactly where the interaction occurs is very large, because of a rule of quantum mechanics called the Heisenberg Uncertainty Principle. That uncertain zone--the distance it takes for the electron and the gamma ray to assume separate identities--is as long as a millimeter in some cases.

If lots of other atomic nuclei are around, as is the case in dense materials, they can disturb the electrons while they are in the uncertain zone. That suppresses the reaction and reduces the number of gamma rays that otherwise would emerge.

Other workers have detected the LPM effect, but only qualitatively. According to Klein, E-146 measured it 100 times more accurately.

The team's electron beams streamed into thin targets of carbon, aluminum, iron, tungsten, gold, lead, and uranium. The decline in gamma rays was barely noticeable with carbon, which is not dense. But as the materials got heavier, the effect grew more pronounced. For all elements, the results matched those predicted by the Russian theorists very closely.

E-146 marked the first successful use of a "parasitic beam" at SLAC. In a normal run, thick absorbers strip stray particles from the accelerator beam and dump them, removing about 10 percent of all particles. Klein and his colleagues devised a way to capture those particles and convert them into the high-energy electrons they needed.

"We were approved for two weeks of dedicated beam time at SLAC for this project, but we didn't need it," Klein says. "Effectively, we got something for nothing." Other physicists hope to adopt this approach more often at SLAC.

E-146 required virtually no new equipment. The major energy- measuring device, for instance, was built by UCSC researchers in 1984. Most electronics and other components dated from the 1970s and 1980s. Team members proposed the project in June 1992, were testing the beam by January 1993, and took data in March and April 1993. They discussed results at meetings last summer and now are writing papers for physics journals.

Collaborators include researchers from SLAC, Lawrence Livermore National Laboratory, and American University. Klein's coworkers at UCSC are postdoctoral researcher Linda Kelley and former research physicist Matteo Cavalli-Sforza.

Editor's note: Klein is available for comment until May 27. To reach him, call (408) 459-3457 or send email to spencer@scipp.ucsc.edu.

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