Purdue News

March 3, 1995 Physicists Report Discovery Of Top Quark WEST LAFAYETTE, Ind. – Two teams of Purdue University researchers are among more than 800 scientists from 70 institutions worldwide who Thursday (3/2) announced the discovery of the top quark, a fundamental building block of matter. Two separate experiments at Fermilab – the U.S. Department of Energy's Fermi National Accelerator Laboratory in Batavia, Ill. – have independently found overwhelming evidence for the top quark, a subatomic particle that has eluded detection for two decades. The two teams of experimenters are the Collider Detector at Fermilab (CDF) collaboration and the D-Zero collaboration. "Purdue researchers have contributed to the top quark search for nearly 20 years," said David S. Koltick, professor of physics at Purdue and a member of the D-Zero collaboration. "The search has been a long one, but it has brought phenomenal progress to the field of high-energy physics." Arthur F. Garfinkel, professor of physics and a member of the CDF collaboration, said, "There are only two experiments in the world that can observe the top quark, and Purdue is involved in both of them. It's important for one experiment to check the results of another." Purdue is one of only six institutions worldwide with scientists in both research groups. Scientists in both groups have been eager to find and study the top quark because its discovery would further complete what's called the Standard Model, the prevailing theory describing the particles and forces that determine the fundamental nature of matter and energy. All matter in the universe is made up of subatomic particles called quarks and leptons. Protons, neutrons and all nuclei in atoms are composed of quarks. By 1977, all six quarks that were predicted to exist had been detected – except the top quark. Since then, Fermilab has increased its capabilities so that top quarks can be created by high-energy collisions between beams of protons and antiprotons, their antimatter counterparts. The beams are accelerated by a particle accelerator to close to the speed of light. When the beams collide, a top quark and its antimatter partner are produced together in what physicists call an event. The top quark breaks up almost immediately into other particles, including its "partner," the bottom quark. Extremely sensitive and complex detectors carefully track the debris and measure the energy produced in the collision, feeding the information into high-speed computers. Although the top quark exists for only a tiny fraction of a second and cannot be "seen" directly, scientists can infer its creation by analyzing the debris and energy produced in the collision. The CDF and D-Zero collaborations worked independently and used different types of detectors to achieve their results. CDF researchers last April announced that they had found the first direct experimental evidence for the top quark, but stopped short of claiming its discovery. "What we published last year was the first evidence for the top quark, but it wasn't overwhelmingly convincing," said Virgil E. Barnes, professor of physics at Purdue and a member of the CDF group. "Now the word is 'observation,' which is a firmer statement scientifically. We have seen the top quark in an extremely convincing way." Purdue's CDF researchers now estimate that there is less than one chance in a million that their results could be in error. Their previous data yielded a 1-in-400 chance of being incorrect. Since April, the CDF experiment has gathered three and a half times more data and has made major improvements to a device that detects the bottom quarks left over after a top quark break-up. Daniela Bortoletto, assistant professor of physics at Purdue and a member of the CDF collaboration, said the group's new measurement of the top quark's mass – made from analyzing the bottom quark debris – is consistent with both last year's results and theoretical predictions. The D-Zero experiment, named for the location of its detector on Fermilab's accelerator ring, also found the top quark's mass to be consistent with theory, Koltick said. "The top quark is very massive for a subatomic particle – it weighs as much as an atom of gold," he said. Using data collected over the past four years, the D-Zero group identified 17 possible top quark events. Koltick said identifying such a large number of events is significant because it is much larger than expected from random fluctuations in the background, which can mimic top quark decay. "An enormous amount of energy is produced when the beams of protons and antiprotons collide," Koltick said. "From that energy we can determine whether or not we've created a top quark. We estimate that there is a one-in-a-million chance that our observations are due to background noise." Koltick said the D-Zero collaboration now will begin studying the properties of the top quark by using a new optical fiber technology that he and his Purdue colleagues have developed. In addition to Koltick, the other member of the D-Zero collaboration at Purdue is postdoctoral fellow Yuriy Pischaluikov, who worked on top quark research in Russia before coming to Purdue. In addition to Barnes, Bortoletto and Garfinkel, current Purdue members of the CDF collaboration involved with the top quark analysis are postdoctoral fellows Alvin Laasanen, Mark Shaw and Qifeng Shen; and graduate students Mark Kruse and James Tonnison. Purdue News Service: (765) 494-2096; purduenews@purdue.edu