Physicists may have found 'top' quark

Particle physicists appear to be poised for a breakthrough in their quest for the underlying structure of matter. Puzzling phenomena have appeared at energies where present theory predicted there was little left to uncover.

This indicates that reseachers may have come across an unsuspected, and possibly rich, field in which to make new discoveries.

Also, a team at the European Center for Nuclear Research (CERN) at Geneva may have found the long-sought ''top'' quark. Protons, neutrons, and related particles are believed to be made up of combinations of more basic entities called quarks.

Five quarks are already known; a sixth is strongly suspected to exist.

Carlo Rubbia of Harvard University, a member of the CERN team that has ''seen'' the top quark, considers the evidence too thin for a formal announcement.

Speaking informally, however, he says he believes they have found it and that its mass, in terms of equivalent energy, is 40 billion electron volts (Gev).

An electron volt is the energy an electron gains when it is accelerated by a voltage of 1 volt.

The most powerful working accelerator facility, the CERN machine - in which protons and antiprotons collide at 540 Gev - can at best give only hints of phenomena at higher energies.

The 540 Gev is partitioned among a number of quarks and other particles when a proton and antiproton collide.

It is such hints of unsuspected phenomena at energies of perhaps 150 to 200 Gev that now tantalize particle physicists.

These phenomena have appeared mainly at CERN and, to a lesser extent, in early experiments at the Fermi National Accelerator Laboratory (Fermilab) as a new accelerator there is being put into service.

Physicists have thought their so-called ''standard model'' covered phenomena at energies of several hundred Gev. This model includes the theory of quarks and a theory which unifies the electromagnetic and weak forces. The weak force is involved in some forms of radioactivity.

Physicists thought they had put the capstone on this model last year when CERN teams confirmed the existence of three particles predicted by electroweak theory - the positive and negative W and the Z particles.

However, as Dr. Rubbia and his CERN colleague Jean-Marc Gaillard told a recent American Physical Society meeting, some proton-antiproton collisions are producing jets of particles that are too energetic or otherwise can't be accommodated by electroweak theory.

This hints at new and unsuspected phenomena.

At the May 4 centennial celebration at Harvard's Jefferson Physical Laboratory, Rubbia said: ''My own feeling is that we're seeing there something complicated.'' He added that he believes there is a lot of new physics to be discovered at energies of a few hundred Gev. This is just the energy range that will be fully available when Fermilab's 2,000 Gev proton-antiproton collider comes on line over the next few years.