In search of the elusive top quark.

Of the six quarks and six leptons postulated in the standard model of particle physics as the constituents of matter - ordinary and otherwise - only the top quark and the tau neutrino remain unobserved. Researchers working with the CDF detector at Fermilab's Tevatron collider in Batavia, Ill., now conclude that the top quark, if it exists, most likely has a mass greater than 91 billion electron-volts (91 GeV). They see no evidence in their searches to date of a top quark in any of the billions of observed collisions between protons and their antimatter counterparts. The 268 members of the CDF collaboration report the new lower limit on the top quark's mass in the Jan. 27 PHYSICAL REVIEW LETTERS.

By observing the numerous ways in which a particle known as the Z [degree] can decay into other particles, another huge group of researchers working at the Large Electron-Positron collider at the European Center for Particle Physics in Geneva, Switzerland, has tentatively set an upper limit of roughly 190 GeV on the top quark's mass. If its mass proves less than about 170 GeV, the Fermilab team may have a chance of catching a glimpse of this elusive particle by 1994.

Theorists agree that the top quark must exist for the standard model of particle physics to remain viable. The theory requires that quarks come in pairs, and only the bottom quark has no partner so far. "The fact that they come in pairs is an integral part of the theory," Fermilab's John E. Huth said at the American Association for the Advancement of Science's annual meeting, held last month in Chicago. "If you were to find just one quark, with the other one missing, the whole theory falls apart."

At the same time, the standard model in no way specifies what mass the top or any other quark should have. The bottom quark, heaviest of the known quarks, has an experimentally determined mass of about 4.5 GeV, nearly five times that of a proton. The newly established lower limit on the top quark's mass appears to set it apart from the others. "Although we don't have a good [theoretical] understanding of what any of the masses of the fundamental constituents are," says Chris Quigg, also of Fermilab, "the top is now so much heavier than any of the others. . . . It may be the special key to the masses of the other particles."

"Finding the top quark and measuring its mass are important," he adds. "The mass of the top quark is a crucial unmeasured parameter of the standard model."