Final building block of matter, the top quark, may have been sighted Hopkins team plays role in painstaking hunt at Fermilab

December 28, 1992|By Douglas Birch | Douglas Birch,Staff Writer

After an epic 14-year hunt, scientists think they may have harpooned the Great White Whale of physics -- an elusive particle called the top quark.

If so, Johns Hopkins physicist Bruce A. Barnett and his graduate students can claim credit for helping build the high-tech harpoon gun.

The top quark is the last of the fundamental building blocks of matter to have its existence confirmed in the laboratory. If the latest findings hold up, the event could become one of the most celebrated in the study of particle physics -- an exotic world so tiny that it can only be reached by mathematics, imagination and instruments that weigh 4 million pounds and produce billions of electron volts.

"It's the most important thing we're doing right now," Dr. Barnett said in an interview last week. "There isn't any question. It's the main missing link."

At first, the hundreds of Fermilab collaborators who spotted the quark two months ago did not want to discuss the finding without studying the data further and confirming them by detecting a second top quark.

But there was too much excitement to keep a lid on the news. Faxes and electronic mail messages shot across the world.

"There was tremendous pressure on the collaboration to tell about the event," Dr. Barnett said. "Everybody was harassing us, asking so many questions."

The physicists have decided not to claim victory until they confirm the findings. They want to make sure that the footprints of the top quark that appeared on their screens weren't just a glitch in their proton-antiproton collider, called Tevatron.

"We really need more data to make a positive claim, to stake our reputations on it," Dr. Barnett said.

But speaking for himself, the Hopkins physicist sees little room for doubt. "I don't know personally of any other good explanation for this event," he said.

Meanwhile, another team at Fermilab near Chicago and a third in Europe are also in the race to find the top quark. So Dr. Barnett's colleagues are still working in shifts around-the-clock, seven days a week, trying to repeat what they are modestly calling their "interesting event." So far, they haven't succeeded.

"Depending on what the mass is, it may be months before we get another one," Dr. Barnett said.

According to the leading theory of particle physics, called the "standard model," there are 12 fundamental constituents of matter.

Six of them are lightweight leptons -- a clan that includes the atom's orbiting electron and the neutrino, a ghostly particle with nearly no mass.

The other six are the quarks that make up the heavy objects in the atom's nucleus, such as the proton and electron.

Quarks come in six varieties, whimsically named "up," "down," "strange," "charm," "top" and "bottom." A proton, for example, consists of two "up" quarks and one "down" quark.

Since 1977, when the bottom quark was discovered, only one particle remained stubbornly immune to discovery by physicists using particle colliders -- the top quark.

That's probably because the top quark is far larger than other quarks and takes much more energy to produce. Another problem is that top quarks don't hang around very long -- only a millionth of a millionth of a millionth of a second. Then they decay into other particles.

Colliders such as Fermilab's don't just blast atomic particles into pieces. They create new, larger bits of matter through high-energy collisions, in accordance with Einstein's formula that says matter and energy can be changed into one another.

At 3 a.m. on Halloween, Fermilab's Tevatron collider sent streams of protons and antiprotons racing in opposite directions at 99.99 percent of the speed of light around a circular, underground track. An antiproton is the antimatter version of the proton, and when they touch, they destroy one another.

"It's the highest energy machine mankind has managed to make," Dr. Barnett said.

As had happened perhaps a trillion times before over the previous six months, the protons and their antimatter counterparts struck head-on, annihilating each other.

As usual, the crash created a shower of new particles. But for the first time, the impact produced four flying specks of matter thought to be the telltale signs of a top quark's decay -- two bottom quarks, an electron and another lepton called a muon.

Dr. Barnett's team built the Tevatron's "vertex detector," a machine the size and shape of a small fire extinguisher that sits in the center of the 2,000-ton apparatus known as the Collider Detector Facility.

The vertex detector is an onion of silicon sheets designed to track very accurately the path of the short-lived quarks.

The vertex detector traces the bottom quarks produced by the top quark's almost instantaneous decay. The bottom quarks grow old more gracefully, traveling perhaps a millimeter before they, too, decay into other particles.

"You put the detector as close as you possibly can to where the protons-antiprotons are colliding," Dr. Barnett said.

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