Building matter from a dozen blocks.

Building Matter From a Dozen Blocks

The zoo fundamental particles that make up ordinary matter appears to have a limited capacity after all. Last week, five separate teams of researchers reported evidence that, taken together, limits to three the number of families of fundamental particles known as quarks and leptons that can exist.

"The evidence is very impressive," says Jonathan M. Dorfan of the Stanford (Calif.) Linear Accelerator Center. According to the so-called standard model of particle physics, each family is represented by a particular type of neutrino -- a massless particles that interacts only feebly with matter. "The likelihood of having a fourth generation characterized by a massless neutrino looks particularly bleak," Dorfan says.

The findings, reported by the Mark II detector team at the Standord Linear Collider and by the four detector groups at the large Electron-Positron (LEP) collider in Geneva, Switzerland, mark a milestone in attempts to understand the structure of matter. They confirm earlier astrophysical calculations that had constrained the number of massless or light neutrinos to four or fewer -- a conclusion based on the apparent abundance of helium in the universe.

The results also show that the standard model works quite well. That model assumes ordinary matter consists of two types of particles, quarks and leptons, and that the forces between them are carried by particles known as bosons. There are three types of leptons (electrons, muons and tau particles), each associated with a different type of neutrino, and there are three pairs of quarks (one pair in each family), which combine to make up protons, neutrons and other heavy particles.

The latest experimental results come from observations of the decay of Z[degree] particles -- massive particles created in collisions between high-energy electrons and positrons (the antimatter counterpart of electrons). The new particles promtply decay into leptons or pairs of quarks, which decay further, producing a spray of debris.

By examining how the Z[degree] decays into other particles, physicists can tally the number of types of neutrinos that take part in the decay process. The number of neutrino types affects the form of the bell-shaped curve that represents the number of Z[degree] particles produced plotted against the collision energy. Any extra families of fundamental particles would widen the curve and lower the height of its peak.

All five research groups produced data consistent with the existence of only three types of massless neutrinos, and hence, three families of fundamental particles. But that doesn't eliminate the possibility of finding new particles -- for example, those associated with heavy neutrinos (if such particles exist). However, the chances of finding a massive neutrino now appear slim, several physicists suggest.

For the next two years, researchers, especially those using LEP, intend to focus on a search for the Higgs boson, postulated more than 30 years ago as a way to explain why different particles have the particular masses they do. If the Higgs boson exists, it would show up only rarely in Z[degree] decays, perhaps once in a million events. So far, LEP has produced 11,000 Z[degree] particles and Stanford has produced 500.

"It's a very rare event, so you need a lot of data," says George A. Snow of the University of Maryland at College Park, who works with one of the LEP detectors. "But something wild may happen that lets us find the Higgs particle sooner." Variations on the Higgs model suggest the possibility of several different types of Higgs particles, which may show up more readily in Z[degree] decay data.

"There's still physics to do," Dorfan says. Although one crucial question has been settled, many more remain, including the basic one of why there are exactly three families of fundamental particles making up ordinary matter.