A Colossal Collision Course

By MICHAEL D. LEMONICK

Hundreds of feet beneath the ground outside the Swiss town of Meyrin, near Geneva, a six-year, $660 million construction project is rushing toward a payoff. Workers at the European Center for Particle Physics (CERN) have excavated a 12-ft.-wide circular tunnel that is 16 miles in circumference, installed nearly 5,000 powerful electromagnets, and put along the ring four massive detectors, each weighing several tons but sensitive to the passage of a single subatomic particle. This week, if all goes according to plan, technicians will begin test runs of the largest scientific instrument in the world.

Called the large electron-positron collider (LEP), it will smash together electrons and positrons -- "antimatter" particles that are similar to electrons except that their charge is positive rather than negative. From the debris of the collisions, which involve particles traveling at nearly the speed of light, physicists hope to get information that will solidify -- or upset -- their understanding of the fundamental building blocks of matter and energy. Says Carlo Rubbia, CERN's director general: "This is the main road in basic science. You never know where the main road is really going to take you." Agrees Steven Weinberg of the University of Texas, a Nobel prizewinner in theoretical physics: "Maybe we will discover some weird particle for which there is no experimental evidence, and that would open a whole new chapter in elementary-particle physics."

But while the unexpected is always possible, CERN physicists do have a specific quarry to start with. As soon as the LEP has been put through its paces, they will begin taking a hard look at a particle called the Z 0, which will emerge in great numbers from the electron-positron collisions. The discovery of the Z 0 and two related particles, W+ and W-, in 1982 and 1983 won a Nobel for CERN scientists Rubbia and Simon van der Meer. The three particles carry the weak nuclear force, one of the four fundamental forces of nature, which is responsible for radioactive decay.

What interests physicists, though, is not just what the Z 0 does but how long it lives before decaying. The precise length of its life, which is less than a trillionth of a trillionth of a second, will reveal how many sorts of particles the Z 0 decays into and thus how many other particles exist. Current theory says there may be only one fundamental particle of matter, called the top quark, left to observe. But there may be many more, and gauging the Z 0's lifetime will tell physicists how close they are to a full understanding of the particle menagerie.

The quest for that measurement has become a tight race between European and U.S. physicists. With the new LEP, the Europeans are confident that they can win, but they will have to hurry. A U.S. accelerator called the Stanford linear collider (SLC), built in a hurry (3 1/2 years) and on the cheap ($115 million), has been struggling since February to measure the Z 0. Despite delays in getting the machine up and running, physicists at the Stanford Linear Accelerator Center, in California, have already produced 120 Z 0s. That is enough to calculate the particle's mass more accurately than ever before. And by the end of the year, when the number of Z 0s produced is expected to reach 2,000, the Stanford scientists think they could have the Z 0's life expectancy pinned down. But by that time the LEP may have beaten them to the goal. "I had hoped," acknowledges Burton Richter, the Stanford center's director, "that we would be in the shape we are now in a year ago. If a miracle had happened and we'd come on two years ago, then we'd have scooped up a lot of Z 0 physics." Now he and his colleagues can only hope the LEP will have extensive start-up problems of its own.

Many experts think that is unlikely. Richter bet on a new approach to accelerator design, sending the positrons and electrons down a two-mile-long straight track, then spinning them out in opposing semicircles before colliding them. The CERN machine is more conventional and thus more likely to work from the start. The positrons and electrons in the LEP are made to circle repeatedly in opposite directions through the tunnel, with new particles added periodically to the stream. In a given period of time, the LEP is expected to produce hundreds of times as many Z 0s as the Stanford collider does. That gives CERN the best odds of being the first to measure the Z 0's life-span.

Although CERN's staff tries to be diplomatic about competition with the U.S. in particle physics, there is little doubt that the LEP has given the Europeans a major advantage. "I don't think of science as a football game," says Ugo Amaldi, who oversees one of the LEP's detectors. "But if you look at the number of American scientists coming here, it is clear that our way of doing things is attracting interest."

That is not to say that the U.S. is second rate. The Tevatron, an accelerator at Fermilab, near Chicago, that smashes together protons and antiprotons, is still the most powerful collider in the world, and the proposed superconducting supercollider, planned for Texas, will be more powerful still. Proton-antiproton collisions entail more energy than electron- positron collisions and thus are more likely to generate previously undiscovered particles. But proton-antiproton impacts generate more subatomic debris, which makes it harder to study the properties of individual particles carefully. For what Amaldi calls "precision physics," Europe could soon be No. 1.

With reporting by William Dowell/Geneva and J. Madeleine Nash/Palo Alto