The top bottoms out – again – ‘top’ quark still defies detection – Physics – 1992
David H. Freedman
AT THE HEART OF MATTER, physicists believe, are electrons and quarks. Electrons were detected by J.J. Thomson 95 years ago, but quarks are another story. They normally don’t exist as single, free particles; instead they glom together in groups of two or three to form familiar protons or other particles. Try to separate quarks, and the glue constraining them gets stronger, springlike, as the distance between them increases. Even colossal particle colliders can’t free them up.
These same colliders, however, can create quarks from the burst of energy released by demolished protons–a transaction described by Einstein’s E = [mc.sup.2], which states that energy can be converted into mass, and vice versa. According to the currently accepted model, the quark family should have six members, somewhat whimsically named “up,” “down,” “strange,” “charm,” “bottom,” and “top.” Over the past two decades, five have left their tracks in the debris of particle collisions. But the “top” has yet to make an appearance. Twice its discovery has been trumpeted, only to be retracted months later. In 1985 the news that the top had been found by scientists at the giant CERN accelerator in Geneva was announced in the New York Times. And this past May word spread that a team working at the Fermi National Accelerator Laboratory in Batavia, Illinois, had the top quark nailed. But the physicists running the project demurred; their evidence for a top-quark find was far from convincing.
Given the hundreds of millions of dollars’ worth of accelerators, detectors, and computers used in the search, you’d think physicists would know a top quark when they see one. Alas, it’s not that simple. Quarks flicker into existence far too briefly to leave more than the merest trace. Instead, physicists must infer their passing from clues–like backwoods trappers searching for a trampled leaf here, some gnawed bark there. Unfortunately, picking out these clues amid the sea of similar footprints left by electrons, muons, pions, and other particles produced in an accelerator is an overwhelming task. “It’s not like you come in on the afternoon shift and say, ‘Aha, there it is,’ and the next day you’re writing a paper,” explains Melvyn Shochet, one of the leaders of the Fermilab top-quark collaboration. Physicists can argue for years about whether a particular set of tracks is the authentic calling card of a sought-after particle.
The top quark’s signature is so muddled it can make a grown physicist weep. Other quarks pop into existence in neat pairs with their antimatter counterparts–charm and anticharm, for example. The pairs disintegrate in characteristic patterns, leaving a clean pair of electrons, say, in their wake. But the top quark is at least 60 times heavier than the charm. Such heavy top-antitop pairs do not disintegrate cleanly. Each partner decays separately, leaving a messy spaghetti of tracks that could be the sought-after signature; however, an imposter could easily produce a similar set of signals. “It’s very frustrating,” sighs Peter McIntyre, a member of the Fermilab team.
It’s not impossible, though. A “farm” of high-powered computers at Fermilab can sift out most of the background noise. Still, the chance that a promising-looking signature is that of the top quark is at best just better than fifty-fifty. To eliminate look-alikes, physicists watch for consistency; fakes could take many different forms, while the true top should leave the same signature repeatedly. The rumor that Fermilab had found the top quark sprouted from one such “possible” set of tracks. “That rumor was wrong,” growls Shochet. “You’d probably need to see 10 to 15 of them to be conclusive.”
That could take a while. For one thing, it can take the computers a year to plow through the tracks left by the billions of particles produced in one year’s run. For another, the top quark is so massive that even Fermilab may not have the energy to produce it. A series of scheduled enhancements should boost the physicists’ chances, however. “If we can’t see it at that point, something’s seriously wrong with physics,” says Shochet. At that point, physicists would have to transfer their hopes to the projected 40-trillion-electron-volt Superconducting Supercollider, which shouldn’t have that much trouble producing a top quark even more massive than the upper boundaries allowed by current theory. Or who knows? Maybe the top quark will have the last laugh once again.
COPYRIGHT 1993 Discover
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