Purdue News

April 26, 1994 Physicists Find New Evidence In Search For Top Quark WEST LAFAYETTE, Ind. – Three Purdue University physics professors and several Purdue students and postdoctoral fellows are among more than 400 researchers from 35 institutions who today (Tuesday, 4/26) announced new results in the search for the top quark. Physicists at the U.S. Department of Energy's Fermi National Accelerator Laboratory in Batavia, III., presented the first direct evidence for the top quark, a subatomic particle that is the last undiscovered quark of the six predicted by current scientific theory. Scientists worldwide have sought experimental evidence for the top quark since the discovery of its "partner," the bottom quark, at Fermilab in 1977. "For nearly 20 years, scientists have been eager to find and study the top quark because its discovery would be a wonderful completion of what's called the Standard Model," said Virgil E. Barnes, a professor of physics at Purdue and a member of the collaboration. "The Standard Model is the prevailing theory describing the particles and forces that determine the fundamental nature of matter and energy." Based on the new evidence, researchers stopped short of claiming discovery of the top quark. "It's not quite an official discovery, but it's clear we have found strong evidence for the top quark," Barnes said. All matter in the universe is made up of subatomic particles called quarks and leptons. Six of each type are thought to exist. Protons, neutrons and all nuclei in atoms are composed of quarks. A research paper submitted to The Physical Review by the 440-member Collider Detector at Fermilab (CDF) collaboration presents evidence for the production of top quarks in high-energy collisions between protons and antiprotons, their antimatter counterparts. The experiments were carried out at Fermilab's Tevatron particle accelerator. At Fermilab, top quarks can be created by the energy released in collisions between beams of protons and antiprotons that are accelerated by the particle accelerator to close to the speed of light. In such collisions, a top quark and its antimatter partner are produced together in what physicists call an event. The top quark decays almost immediately into other particles. The final product manifests itself as a column, or jet, of particles in a detector. The information from the collision is fed into high-speed computers, which perform additional analyses. In addition to taking and analyzing data from the experiment, Purdue researchers have been instrumental in the design and construction of the collider detector at Fermilab. Purdue's affiliation with Fermilab began in 1978, said Arthur F. Garfinkel, a member of the collaboration who has been a professor of physics at Purdue since 1967. "A major part of the detector was fabricated at Purdue and assembled at Fermilab, including the large calorimeter used to detect particle energy," said Garfinkel, who, with Barnes, worked on the original design. "The calorimeter consists of four arches, each with 12 pieces, and each arch weighs about 13 tons. In the 1980s, Purdue faculty and students were involved in this detector's construction, and they continue to work with the large international collaboration on upgrades of the equipment and research." The Fermilab investigators used data collected between August 1992 and June 1993 to perform three searches for the top quark. During this data run, approximately one trillion proton-antiproton collisions occurred in the collider detector. Of these, 16 million were recorded and later fully analyzed. Among these are the 12 top quark candidate events reported today. Purdue researchers said identifying the 12 possible top quark events is significant because the number of candidates is larger than expected from the "background," which can mimic top quark decay. "Using a device called a silicon vertex detector, we were able to reduce the background level to enhance the events that were more interesting," explained Daniela Bortoletto, assistant professor of physics at Purdue and a member of the collaboration. "We didn't have this detector in previous data taking runs. It has given us a more powerful handle on picking the needle out of the haystack by making the signal stand out more clearly." The collaboration's best estimate is that there is roughly one chance in 400 that the effect was due to random statistical fluctuations in the background. Although this appears to be very small, scientists generally require a smaller probability of a background fluctuation before a new phenomenon is firmly established. "The odds of nature fooling us are pretty slim," Barnes said. "On the other hand, for a new discovery, people require very good odds, and one in 400 isn't quite good enough to make it official." The researchers expect data-taking now under way at Fermilab to yield enough additional events in the next 15 to 24 months to confirm the top quark's existence and to allow more precise measurements of its mass. "We first have to be absolutely certain this is not just a fluctuation in the background," Garfinkel said. "Once we have enough tops to really pin down its mass and other properties, then we will have clues that may help us understand the process by which all objects acquire mass." Despite intensive searches at accelerator laboratories in Europe, Japan and the United States, the top quark has eluded discovery because of its large mass in comparison to other subatomic particles. The more massive a subatomic particle, the more energy is required to produce it in collisions, and the harder it is to find. "This is only the first chapter in a long book," Garfinkel added. "The study of the properties of the top quark is really the beginning of a new era in particle physics. That's quite exciting." In addition to Barnes, Bortoletto and Garfinkel, current Purdue members of the CDF collaboration are postdoctoral fellows Alvin Laasanen, Mark Shaw and Qifeng Shen; and graduate students Mark Bailey, Adam Hardman, Kara Hoffman, Mark Kruse and James Tonnison. Besides U.S. scientists, the CDF collaboration includes physicists from Italy and Japan, as well as Canadian and Taiwanese researchers. Fermilab was established in 1967 as a high-energy physics research laboratory. It is operated by Universities Research Association Inc. under contract with the U.S. Department of Energy. Purdue News Service: (765) 494-2096; purduenews@purdue.edu