Putting limits on the top quark.

Putting limits on the top quark 

Particle physicists have so far found evidence in the debris from particle collisions for five species of quarks: up, down, strange, charm and bottom. But the so-called standard model of particle physics predicts a sixth species: the top quark. Researchers working with a detector at the Fermi National Accelerator Laboratory's Tevatron collider in Batavia, Ill., now conclude that the top quark, if it exists, most likely has a mass greater than 89 billion electron-volts (GeV) but less than 250 GeV. They see no trace of the telltale signals that would reveal the production of a top quark in any of 200 billion collisions between protons and their antimatter counterparts during last year's Tevatron expriment.

Physicists are puzzled by why the top quark should have such a large mass. At roughly 0.4 GeV, the up and down quarks, from which one can build protons and neutrons, are relatively light. The proton mass itself is a little less than 1 GeV. The strange quark has a mass of about 0.5 GeV, the charm quark about 1.5 GeV and the bottom quark about 4.5 GeV. The apparent trend of heavier quarks being three times more massive than their predecessors suggests that the top quark ought to have a mass around 13.5 GeV. But every search so far has found no top quark, pushing its expected mass ever higher.

If the top quark has a mass of 130 GeV or less, then researchers at Fermilab may yet catch a glimpse of this elusive beast. "This puts the top quark within the reach of the Tevatron if proposed upgrades are implemented," says Fermilab's Hans B. Jensen.

And then there were three

By observing the many ways in which a particle known as the Z[degree] can decay into other particles, researchers working with the ALEPH detector at the Large Electron-Positron collider in Geneva, Switzerland, have now obtained the msot stringent limit yet excluding the existence of a fourth generation of elementary particles. Their results firmly establish that the elementary particles of matter fall into only three sets, each one characterized by a particular type of neutrino (SN: 10/21/89, p.260).