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08/21/92

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SLAC begins research with polarized electrons

STANFORD -- The Stanford Linear Collider (SLC), has begun generating scientific results using a polarized electron beam.

Physicists working at the Stanford Linear Accelerator Center (SLAC) reported results, on the production of Z particles using a polarized electron beam, at the 26th International Conference on High-Energy Physics, held in Dallas earlier this month.

SLAC Director Burton Richter said he was pleased that the more than two-mile long device, the world's first linear collider, is finally performing close to expectations, producing scientific results that cannot be duplicated by other machines.

"We've worked many years to develop this capability," he said. "The hundreds of scientists, engineers and technicians who have pioneered this revolutionary new particle collider can be very proud of their efforts."

The Stanford Linear Collider was upgraded in April to generate a high-energy beam of polarized electrons whose spins are aligned with the beam axis. SLAC physicists clash this beam with an unpolarized beam of positrons (antipar ticles of the electron) to produce Z particles - the heaviest elementary particles yet discovered - at the collider interaction point.

No other particle collider in the world can create these rare, massive particles using a polarized beam, which gives physicists an added degree of control over the experimental conditions. The 17-mile LEP ring at the European C enter for Particle Physics in Geneva, Switzerland, makes Z particles by colliding unpolarized beams of electrons and positrons.

The Stanford collider has performed well since June, producing a total of more than 13,000 Z particles - often exceeding a thousand per week.

"We've made many major improvements during the past year," said Nan Phinney, leader of a team of physicists working on the collider, who reported on their recent progress at the Dallas meeting. "That's a big reason the beams ar e now colliding about two-thirds of the time on an average day."

So far, a group of about 150 physicists led by Charles Baltay of Yale University and Martin Breidenbach of SLAC has observed and recorded more than 10,000 Z particles on magnetic tape - surpassing the goal set for the current e xperimental run with more than a month left to go. The team, representing 36 universities and laboratories from the United States, Canada, Great Britain, Italy and Japan, has spent the past six years designing and building the four kil oton Stanford Large Detector (SLD) used to observe the Z particles disintegrate.

The analyses presented at Dallas were based on about 5,000 events. One early result to emerge from the detector experiments is a first-ever study of how often the Z is produced when the electron beam is polarized along its dire ction of travel versus the opposite way. Physicists compare what occurs when the electrons "rotate" clockwise against counterclockwise rotation about the beam axis.

Such a comparison yields a new measurement of the so-called Weinberg angle, a key parameter of the dominant Standard Model of particle physics. Its value determines the degree of mixing between the familiar electromagnetic forc e and the weak force that is responsible for radioactive decay.

Polarized electrons are created by shining a powerful beam of polarized laser light on a specially prepared surface of gallium arsenide - a semiconductor that is finding increasing use in nearby Silicon Valley. These electrons collect into compact bunches and reach an energy of about 46 billion volts in the Stanford Linear Collider. Maintaining their level of polarization during transit, has proved to be surprisingly easy. Losses of only a few percent are no rmally encountered.

Changing the polarization of the electrons (from clockwise to counter-clockwise, or vice versa) is readily accomplished by switching the polarization of the laser light. Subtle effects and differences can be studied using this technique.

With the original goal for the run (of observing 10,000 Z particles) already achieved, the Stanford Large Detector physicists are continuing to log additional events to improve the accuracy of this and other measurements. The r un will end in late September.

The success shows that linear colliders can make important contributions to research in high-energy physics, Richter said.

"SLAC's development of this prototype is a crucial first step in a large-scale, worldwide effort to develop linear colliders into a standard experimental tool by early in the next century," he said.

The Stanford Linear Collider was built from 1983 to 1987, using $115 million in construction funds provided by the U.S. Department of Energy. After a long period of commissioning, it began high-energy physics research in 1989. Since then, steady improvements have occurred in its output of Z particles.

SLAC is a national laboratory operated by Stanford University under contract with the Department of Energy.

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