Over The Horizon

Topping Up Quarks

Physicists at Fermilab in Illinois have announced that they have seen evidence of the top quark, the heaviest known fundamental particle. They have carefully not yet claimed discovery of the top quark, because their results are statistical in nature and the possibility of error, although small, is not negligible. The results, if borne out by further data runs now in progress, tie up a loose end that has been dangling for over a decade: Without the top quark, particle physics theory would unravel.

In the highly successful "standard model" of particle physics, all matter is made out fundamental particles called quarks and leptons. These occur in groupings called generations, the stable lowest generation consisting of the "up" quark, the "down" quark, the electron, and the electron neutrino. Protons and neutrons, the main constituents of the nuclei of atoms, are made of triplets of up and down quarks.

The second generation consists of the "strange" quark, the "charm" quark, the muon, and the mu neutrino. Except for the massless neutrino, these particles are more massive than the corresponding first-generation particles and they decay into first-generation quarks and leptons in a fraction of a second. The third generation consists of the still-heavier "bottom" and "top" quarks (also known as "beauty" and "truth"), the tau, and the tau neutrino. The bottom quark was observed over a decade ago, but its partner the top has remained elusive.

Failure to discover the top quark would be a severe blow because the mathematical consistency of the standard model depends crucially on each generation mirroring the pattern of the lowest generation. Conversely, the top is believed to be the final quark because other experimental results strongly imply that there are only three generations of neutrinos.

The Fermilab results, widely rumoured for months, were detailed in a 150-page paper with approximately 400 co-authors. The experiment collides high-energy beams of protons and antiprotons, each collision producing a spray of particles, some emerging on separate tracks, others in jets of many particles. On extremely rare occasions, such a collision will produce a top quark and an anti-top quark, which almost instantaneously decay into lighter particles. Computer analysis of the emitted particles and jets, their energies, and so on, eliminates all those events that are probably not due to the ephemeral existence of a topanti-top pair.

Trillions of protonantiproton collisions were observed at Fermilab over the period August 1992 to May 1993. A total of 12 events had the signature of the top quark with a mass of about 174 GeV, about 40 times the mass of its partner, the bottom, and about 180 times the mass of the proton.

That, however, is not the end of the story, because even those 12 events may be due to other known processes mimicking the top signal. After laborious analyses, the scientists estimate that there is a 1-in-400 chance that all 12 events are due to such background events, which is why they haven't claimed discovery of the top. In addition, a parallel experiment at Fermilab by a second group has failed to see evidence for the top. Both groups are currently conducting further collisions, which will provide enough data to settle the matter.

Graham Collins, NZSM Washington DC