More than two dozen University of Rochester physicists played a key role in the confirmation of the existence of the top quark, one of 12 basic building blocks of matter that scientists believe make up everything in the universe. The quark's discovery was announced yesterday at the Fermi National Accelerator Laboratory (Fermilab), a high-energy physics laboratory near Chicago.
For more than a decade two separate teams of scientists have searched for "the top." About a year ago one team, the Collider Detector at Fermilab (CDF), announced that it had found strong evidence for the top's existence. Yesterday both CDF and the other experiment, DZero, announced that the top has indeed been discovered.
"With this discovery of the top quark, our overall picture of matter fits together beautifully," says Thomas Ferbel, the professor of physics at the University who led a team of 16 Rochester physicists on DZero. Together more than 800 scientists from around the world work on both DZero and CDF.
Over the past year CDF has added to its evidence, so that now, says Assistant Professor Paul Tipton, "There's only about one chance in a million that we're being fooled by background signals. This is beyond statistical question." Tipton last year was one of the primary authors of the scientific paper detailing CDF's evidence; this year Rochester research associate Richard Hughes is one of three authors detailing the latest CDF work.
The two experiments agree that the top quark weighs in more than expected. CDF's work points to a mass of about 176 billion electron-volts, a mass heavier than an entire atom of gold. DZero points to a mass of about 199. The measurement of the mass is very difficult, says Ferbel, and the margin in the uncertainty makes the two results consistent with each other. Both groups have submitted papers to Physical Review Letters detailing their results.
The top quark is one of 12 fundamental particles from which, according to current theory, all matter is constructed. The top is by far the heaviest of the particles, and that's why it was the last to be observed in the laboratory: the bigger the particle, the more energy needed to create it. At Fermilab, protons and antiprotons are hurled through a four-mile circular accelerator and smashed together. When they collide they release enormous amounts of energy, creating other particles such as the top. The top is so short-lived that scientists detect it by spotting an array of particles that can be created only by the decay of a top quark.
It takes hundreds of scientists to operate the huge detectors needed to observe such tiny particles. On the 5,000-ton DZero detector, Ferbel and Professor Fred Lobkowicz, along with the other Rochester scientists, helped build and operate the calorimeters, devices that absorb the energy of particles such as protons, neutrons, and pions and convert that energy into electrical signals which can be analyzed. Meanwhile, the group led by Tipton produced some of the strongest evidence for the top, leading the construction and data analysis of a silicon vertex detector that was a major upgrade to CDF.
Now that they've sighted the top, Rochester scientists are developing ways to learn more about it. Ferbel and colleagues are developing special optical fibers known as scintillating fibers that emit light when charged particles go through them. "You need thousands of these fibers, which are very thin -- it's like taking a bunch of spaghetti, with each strand carrying an electrical signal. We take those signals and organize them so we can tell which particles the fibers are detecting," says Ferbel.
Besides Ferbel and Lobkowicz, University scientists working on DZero include graduate students Cathy Cretsinger, Hui Zhu, Eunil Won, Dylan Casey, Kathy Fatyga and Kisuk Hahn; research associates Marc Paterno, Stefan Gruenendahl, Jaehoon Yu, Marek Zielinski, Kamel Bazizi and Vladimir Glebov; and former student Sarah Durston-Johnson, now on the faculty at Hobart and William Smith Colleges in Geneva.
Tipton is one of 13 Rochester scientists who have worked on CDF. The others are Professor Arie Bodek; graduate students Phillip Koehn, Kirsten Tollefson, Manoj Pillai, Mark Dickson, and Qun Fan; senior research associates Priscilla Auchincloss, Howard Budd and Willis Sakumoto; and research associates Pawel de Barbaro, Richard Hughes and Brian Winer.
Mechanical engineer Tom Haelen has also played a key role in both CDF and DZero, helping build the detectors that actually observe the particles. tr