Uniting fundamental forces in a new package.

When physicists try to construct a mathematical model aimed at putting into a tidy, compact package all the different ways in which elementary particles interact, they generally start with the simplext possibility. But that strategy fails when applied to the development of a grand unified theory, which combines the present theory governing quark behavior with the electroweak theory describing the interactions of electrons, photons and related particles. In this case, the simplest model founders on its prediction that protons decay faster than indicated by observations of proton stability.

Theorist Paul H. Frampton of the University of North Carolina in Chapel Hill and his colleagues are now exploring an alternative, though more complicated, grand unified theory that seems to avoid some of the inherent problems of the original, simple model. "In this [new] theory, the proton is essentially stable," Frampton says. The model also predicts the existence of particles called leptoquarks, which, in principle, could be detected in a proton-electron collider nearing completion in Germany.

Frampton's prediction appears in a paper submitted to PHYSICAL REVIEW LETTERS, and the model itself was first described in the Feb. 5, 1990 PHYSICAL REVIEW LETTERS.

Like other grand unified theories, the new scheme goes by its mathematical designation, SU(15), which describes the mathematical "group" that acts as a framework for the model. The original grand unified theory was based on a group called SU(5). "It's a different approach to grand unification," says Thomas W. Kephart of Vanderbilt University in Nashville. "It's not a perfect model, but it has some very attractive points."

The idea behind grand unification is that at sufficiently high energies, the strong and electroweak forces lose their identities and merge into a more fundamental interaction. The new model suggests that this unification occurs in several stages rather than in a single step. Each stage necessitates the existence of additional force-transmitting particles. Those associated with the lowest step may exist at such a low energy that an accelerator such as the proposed Super-conducting Super Collider (SSC) would find them.

In addition, the theory predicts that high-energy collisions between electrons and protons would produce leptoquarks. If leptoquarks are light enough, physicists should be able to detect them when experiments begin next year at the Hadron-Electron-Ring Accelerator in Hamburg, Germany.

"The discovery of leptoquarks would be evidence for grand unification as compelling as proton decay," Frampton says. The SU(15) model, however, suffers the drawback of requiring each known quark and every member of the Lepton family (which includes the electron, muon and neutrino) to have a closely related cousin. "It looks implausible that there would be so many [additional] particles--that there is another world sitting just around the corner," says Palash B. Pal of the University of Oregon in Eugene. Nonetheless, if such particles exist, they would probably have masses that put them within the SSC's detection range.

To avoid this hypothetical "particle glut," Frampton and others have begun looking for alternative grand unified theories that preserve the desirable features of SU(15) but don't require the existence of these so-called "mirror fermions."

Indeed, SU(15) doesn't have the grand unification stage all to itself, and not everyone takes it seriously. "It's a complicated story," says Paul Langacker of the University of Pennsylvania in Philadelphia. "I suspect that [Frampton's model] is just one out of a very large class of models that could be constructed."

"SU(15) is an interesting model, though it has problems," says Nilendra G. Desh-pande of the University of Oregon. "It makes some new predictions, and one would like to see how they work out."