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This article is from the In-Depth Report Farewell to the Tevatron

"You're the Top..." [From the Archive]

Fermilab finds the top quark—sort of



FERMI NATIONAL ACCELERATOR LABORATORY

Editor's note: This article was originally published in the July 1994 issue of Scientific American and describes the first tentative sighting of the top quark. Confirmation came a year later. We have resurfaced this article to commemorate the end of the Tevatron.

Rumors had been circulating since last summer, so it seemed to be just a matter of time before an official announcement would be made. When the call came, the expectant media circus descended on the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. Yet the Fermilab speakers hesitated to deliver the media goods. Now, we’re not claiming a discovery cautioned William C. Carithers, Jr., one of the spokespersons for the hundreds of physicists who garnered the results. What we see is the first direct hint that the top quark is thereto Fermilab director John Peoples, Jr., reinforced the hedge: "I assure you, we are going to have far more evidence for it soon."

Certainly the anticipated prize was worth the attention. For two decades, the top quark has been one of the Holy Grails of high-energy physics. Out of the six kinds of quarks thought to make up all matter, it was the only one that had not been observed. The theory that characterizes particles, called the Standard Model, indicates that quarks are organized into three pairs. The first pair includes the up and down quarks, which in different combinations produce protons and neutrons. (The proton contains two up quarks and a down; the neutron grips one up and two down.) The other two pairs consist of the charm and strange quarks, and the top and bottom quarks. These latter pairs make up more exotic, short-lived particles seen in high-energy physics laboratories. Along with quarks, the family of particles known as leptons (neutrinos, electrons, muons and tau particles) composes the elementary constituents of the universe.

The quest for the top quark began after its partner, the bottom quark, was found at Fermilab in 1977. The top quark remained elusive mostly because of its heft (the heavier a particle is, the more energy is needed to create it in accelerators). Early estimates placed that value at a few tens of billions of electron volts (GeV). But when accelerators failed to turn up the top quark, theorists realized the particle must be heavier than they thought.

Scientists had the best shot at finding the top quark once they completed the Tevatron at Fermilab in 1983. The world's most powerful accelerator, it smashes protons and antiprotons together at 1.8 trillion electron volts. At this energy level, physicists believed a top quark should be made once for every few billion collisions. The search demanded the efforts of 440 investigators from 36 institutions, prompting praise for the merits of international cooperation and jokes about the number of physicists needed to install a lightbulb. By 1989 the Tevatron had set a lower limit on the top quark's mass at 91 GeV—a whopping number, considering that the next most massive quark, the bottom, weighed in at only 5 GeV.

To get the results they announced at the press conference, the CDF team members (as they are known, for Collider Detector at Fermilab) collected data from August 1992 to June 1993. According to the Standard Model, a top quark and its antimatter twin could appear in proton-antiproton collisions. The top and antitop quarks would decay into bottom and antibottom quarks and a pair of so-called W bosons. The CDF workers looked for decay products, such as electrons, muons, neutrinos and mesons, of these particles.

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