Colliding positrons, polarized electrons.

The Stanford Linear Collider (SLC) is back in business. Since June, researchers at the facility have been collecting data generated by high-energy collisions between beams of polarized electrons and unpolarized positrons (the antimatter counterparts of electrons).

So far, the international team of more than 150 physicists working with the Stanford Large Detector has observed and recorded more than 10,000 collisions that produced Z particles. These elementary particles are among those carrying the so-called weak force, which is responsible for radioactive decay.

Earlier this month, at the International Conference on High-Energy Physics, held in Dallas, the researchers reported the results of analyses based on data from roughly 5,000 of these events. Their findings yielded a new measurement of a key parameter in the standard model of particle physics, which describes how the fundamental particles of matter interact with one another.

This achievement represents a belated victory for the innovative but trouble-plagued SLC, whose technical glitches cost it the race in 1989 with the 17-mile Large Electron-Positron ring at the European Center for Particle Physics in Geneva, Switzerland, to generate large numbers of Z particles (SN: 9/10/88, p.167; 9/2/89, p.159).

Upgraded in April to produce polarized electrons, whose spins are aligned in the direction of the beam, the SLC now allows researchers to probe more precisely certain kinds of interactions between elementary particles. No other high-energy accelerator in the world has this capability.

Polarized electrons are created by shining a powerful beam of polarized laser light on a specially prepared gallium arsenide surface. The ejected electrons gather into compact bunches and reach an energy of 46 billion electron-volts as they race down the SLC's 2-mile track to crash head-on into high-energy, unpolarized positrons traveling at the same speed.

Researchers can switch the electrons' direction of polarization back and forth between clockwise and anticlockwise by changing the polarization of the laser light. By comparing what happens when electrons "rotate" clockwise versus anticlockwise along the beam axis, Charles Baltay of Yale University, Martin Breidenbach of the Stanford Linear Accelerator Center, and their collaborators could for the first time study how this difference affects the production rate of Z particles.

From these data, they obtained a new measurement of the so-called Weinberg angle. In the standard model, this quantity determines the degree of "mixing" between the electromagnetic and weak forces.