Barnett Leads Hopkins Physicists In Search To Confirm Top Quark By Emil Venere Score another point for particle physics. Last April scientists announced they had found "strong evidence" for the existence of a subatomic particle called the top quark, believed to be the last fundamental building block of matter to have its existence confirmed by research. Now it's official. Last Wednesday two teams of scientists at Fermi National Accelerator Laboratory made a historic statement: physicists have indeed observed the top quark, one of 12 particles from which, according to current theory, all matter is constructed. "This is a scientific milestone of great importance," said Bruce Barnett, a physicist who led a group of seven Hopkins researchers playing an important role in the discovery. "This particle fills in the last of the basic building blocks in the standard model." The standard model says that all matter consists of elementary particles called leptons and quarks. There are six types of quarks, grouped into three sets of "twins" labeled the up and down, the strange and charm, the top and bottom quarks. They are bound together by particles called gluons to make more mundane particles like protons and neutrons. The other elementary particles are six varieties of leptons, a family that includes electrons and their lightweight kin, called neutrinos, as well as their heavier cousins, known as muons and taus. Out of all those constituents, the top quark was the final missing piece of the subatomic puzzle. Not only have scientists found it, they have determined its mass, information that could prove crucial in the study of matter and the forces of nature. But last week's news was accompanied by an element of mystery. The findings support confusing data reported last year: the top quark's mass is about 10 times more than physicists had predicted 15 years ago. "It's certainly surprising that it's this heavy," said Dr. Barnett, vice chairman of the Department of Physics and Astronomy. "It makes life interesting." The findings have implications beyond particle physics. Understanding more about the top quark has a direct relationship to uncovering secrets about how the universe evolved. Many theorists believe that quarks and leptons were all that existed in the very early universe, shortly after the Big Bang of cosmic creation. In time, as the cosmos cooled, protons and neutrons formed, eventually leading to matter as we know it today. To uncover the top quark's role in cosmology, more must be learned about its mass, which can only be done with further experiments. Fermilab, located near Chicago, is the only facility with an accelerator powerful enough to study top quarks, Dr. Barnett said. Hopkins' role in the research is linked to a device called the silicon vertex detector. Developed in collaboration with other American scientists and physicists from Italy, the detector was instrumental in last year's findings. Since then, however, the Hopkins researchers and their colleagues have made major improvements to the device, enabling scientists to collect data critical to the discovery. The detector is situated in the center of an 85-foot-long, 35-foot-high experiment called the Collider Detector at Fermilab (CDF). CDF, in turn, is located at one of the points along a 3.9-mile circular pipe where protons collide with antiprotons, a proton's antimatter equivalent. A system of powerful superconducting magnets called a Tevatron accelerator is used to whip beams of protons and antiprotons around the circular track. 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. Top quarks can't actually be seen, but as they rapidly decay they produce other particles that can be detected. The Hopkins physicists are among 400 scientists on the CDF team, which found the evidence that was announced last April. But at the time those findings were released, a team of scientists working on another Fermilab experiment, called DZero, had not yet found such evidence. More data had to be collected by both teams before researchers could say with certainty that the top quark does exist. Last Thursday, the CDF and DZero teams announced that they had amassed all the data necessary to declare the top quark real. The Hopkins group includes postdoctoral fellows John Skarha and Rick Snider, and graduate students Douglas Glenzinski, Alan Spies, Jeff Tseng and Jeff Cammerata.
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