by James Schultz

Just in time for summer, Jefferson Lab is making ready to accommodate a Ferris wheel. But this version is no run-of-the-mill amusement park ride. Rather, it's one of the most elaborate pieces of research equipment ever installed at the Lab.

Designed to hold a fan-like array of sophisticated detectors built in the U.S., France and Canada, the device's support struts resemble the ride after which they're nicknamed. In turn, the detectors and supports are part of a larger apparatus poised to take center stage when JLab's pending G Zero, or G⁰, experiment begins in Hall C two years from now.

"What we'll be putting into Hall C will be very different from any equipment that already exists. We're looking at a major new installation," says Allison Lung, Lab staff scientist and G⁰ project manager. "G⁰ is a very large, very expensive project that's taken a number of years to get approved and to be fully funded."

In particular, a team of 85 scientists from 18 universities, research institutions and laboratories in the United States and abroad, which is lead by Prof. D. Beck of the University of Illinois, hopes to quantify the contribution of one of the "lightest" of the six "flavors," of quarks: the strange quark. Historically, such measurements have been extremely difficult to conduct because the sought-after effects are quite small.

"This is a challenging project," Lung contends. "The physics is challenging. The actual construction is challenging. The measurement itself is challenging. But the payoff is potentially enormous."

At the heart of the Hall C instrument is a $2 million superconducting magnet in which a liquid hydrogen target will be mounted, and next to it will be positioned eight detector-containing segments. Yet, for all its sophistication, such technology will be a supporting player. Key to the experiment's success will be the demonstrated ability of the accelerator to change the orientation of the spin of the electrons in the beam (their helicity) relative to their direction of motion without changing any other beam properties (such as energy, position, and direction) with remarkable precision. This will allow investigators to measure what physicists call parity violation-the very small (part per million for the experiment of interest) changes in the behavior of a physical system in a situation which is identical except that "left" and "right" have been reversed. A "parity reversed" world is what you "see" on the other side of a mirror, and for the scattering of a beam of polarized electrons, it is a world in which everything is the same except the helicity of the beam has been reversed.

Most forces in nature act the same on "both sides of the mirror," but the subatomic force known as the weak interaction does not. By measuring parity violation in electron-proton scattering, the G⁰ experimenters expect to gain new insights into the role of strange quarks in the proton.

Allison Lung and Paul Gueye, Hampton University user, in the Hall C control room recently, reviewing data for another Hall C Experiment

"It's a very, very tiny effect," Lung says. "You only see it when your electrons are polarized and you look closely at the differences in the number of events your detectors count for each electron helicity. You need, and with this equipment we expect to get, huge numbers of events to get a statistically significant sample. JLab is the perfect place to do this type of experiment because of the high quality of its polarized beam."

By spring 2000, technicians will begin to assemble the entire apparatus in Hall C. By June 2001 all G⁰ components are expected to be in place. Commissioning runs will commence shortly thereafter. Data will be taken two to three months a year for five years.

"This isn't something we're going to put up and take down," Lung points out. "We're going to keep it in Hall C. Meantime, we'll be working on future experimental uses for the equipment. We hope the G⁰ experiment and equipment will have a long and fruitful life."

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