Hopkins Scientists Play Key Role in Top Quark Discovery
Two teams of scientists announced their confirmation of the top quark's existence today at Fermi National Accelerator Laboratory, near Chicago. In April 1994, researchers on one of those teams, working with an experiment called the Collider Detector at Fermilab (CDF), announced the first evidence that they had observed the top quark's tell-tale decay into other particles. Johns Hopkins scientists are involved with the CDF work.
At the time those findings were released, scientists working on another Fermilab experiment called DZero, had no such evidence. Therefore, the scientists needed more data to be certain about the top quark's existence. Since then the physicists have been busy. Hopkins scientists and their CDF collaborators upgraded a key detector and collected two and a half times more data than they did in the first round of experiments.
Now CDF has definitive evidence, with confirmation by DZero scientists; they have indeed observed the top quark, one of 12 fundamental particles from which, according to current theory, all matter is constructed.
"Proving its existence supports the Standard Model of physics, a widely accepted theory about the nature of matter and energy," said Bruce Barnett, one of seven Hopkins researchers involved in the work.
The theory says that all matter consists of elementary particles called leptons and quarks, with the six varieties of quarks grouped into three sets of "twins": the up and down, the strange and charm, the top and bottom. The quarks are bound together by particles called gluons to make the common particles like protons and neutrons.
The other elementary particles are six varieties of leptons, a family that includes electrons and their light-weight kin, called neutrinos, as well as their heavier cousins, called muons and taus.
A device called the Silicon Vertex Detector was instrumental in finding the top quark. Hopkins scientists, working with other Americans and a team from Italy, designed and built this detector. Since last year, however, they have made major improvements to the detector. CDF uses three different techniques in looking for the top quark. The method involving the Silicon Vertex Detector produced a result 100 times more sensitive than the two other methods, enabling researchers to collect data critical to the discovery.
Top quarks can't actually be seen, but as they rapidly decay they produce other particles that can be detected by an array of sophisticated equipment.
The definitive experimental results also support confusing findings reported last year: the top quark is much heavier than its five cousin quarks. It is about 200 times more massive than a proton, which is itself made up of two up quarks and one down quark, said Dr. Barnett, vice chairman of the Hopkins Department of Physics and Astronomy.
Fermilab scientists estimate the top quark's mass at about 176 giga-electron volts (giga is a prefix meaning billion). In comparison, up and down quarks have masses of only a few mega-electron volts (mega means million). Strange quarks are a few hundred mega-electron volts. Charm quarks weigh about 1.5 giga-electron volts (abbreviated GeV), and bottom quarks weigh 5 GeV.
"It's certainly surprising that it's this heavy," Dr. Barnett said. "It's 10 times what one would have guessed 15 years ago."
The Hopkins research team includes post-doctoral fellows John Skarha and Rick Snider, and graduate students Douglas Glenzinski, Alan Spies, Jeff Tseng and Jeff Cammerata.
Hopkins scientists began collaborating in 1989 with CDF physicists to install the Silicon Vertex Detector. It was mounted in the center of the 85-foot-long, 35-foot-high CDF. CDF is located at one of the points along a 3.9-mile circular pipe where the protons collide with antiprotons, a proton's anti-matter equivalent, which has the same mass as a proton but the opposite electrical charge.
The protons and antiprotons are whipped in opposite directions around the circular track by a system of powerful superconducting magnets called the Tevatron accelerator. When they collide they create "jets" of matter's most elementary particles. These particles quickly decay into a wide array of other kinds of exotic constituents, from muons to pions, neutrinos to kaons, which are monitored by a girdle of nearly 100,000 detectors contained in the CDF detector.
The Silicon Vertex Detector sits within an inch of the proton-antiproton collision point. It is a foot-long cylindrical device about 10 inches in diameter. It records the flight of a bottom quark, which scientists believe is produced when top quarks decay. Two of the detectors are connected end-to-end and installed in the CDF. The detectors consist of four layers of silicon and ultra-thin aluminum strips. As particles pass through the silicon they knock electrons free. The electrons are collected on the aluminum strips, causing electronic pulses that are detected and recorded on magnetic data tape. The positions of the pulses are pinpointed, allowing scientists to precisely track the paths made by speeding particles as they stream away from the collision points. Physicists use computers to trace the paths backward to the point of collision, determining the exact collision points and learning more about the specific quarks that caused the spray of particles.
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