By Lynn Yarris, LCYarris@lbl.gov
The long hunt for a crucial missing piece in the scientific picture of matter may be over. An international collaboration of physicists, including a team from the Lawrence Berkeley Laboratory, has announced today the first experimental evidence for the subatomic particle known as the top quark.
The experiment was carried out at Fermi National Accelerator Laboratory's Tevatron, the world's most powerful collider, by the 440-member Collider Detector at Fermilab (CDF) collaboration. Though not claiming a confirmed discovery, the CDF group reports that analysis of high-energy collisions between beams of protons and antiprotons "points strongly" to the production of top quarks at a mass of about 174 GeV (billion electron volts), which is about as heavy as an atom of gold and makes the top quark by far the heaviest elementary particle ever observed.
The CDF group has presented their results in a paper submitted to The Physical Review.
The existence of the top quark is required by the Standard Model, the prevailing theory of the nature of matter. This theory holds there are six quarks, grouped into three distinct pairs, that combine to form composite particles, such as the protons and neutrons in the nuclei of atoms. The fifth of these six, called the bottom quark, was discovered at Fermilab in 1977. Scientists have been searching for its partner, the top quark, ever since.
"We were confident that we would eventually find the top quark," says Lina Galtieri, a physicist who heads the CDF group at LBL. "However, if we had gone much higher without finding it, then we might have to consider whether something was wrong with the Standard Model."
The key to revealing the top quark's existence was the CDF detector, which is actually a 2,000 ton array of more than 100,000 individual particle detectors arranged around a point in the Tevatron where beams of protons and antiprotons collide. This detector system serves as a giant trap from which no known particles (other than neutrinos) or forms of energy can escape.
When a top quark is formed, it promptly decays into a bottom quark and a W boson, which is one of two particles that carry the weak nuclear force. Using data collected between 1992 and 1993 that included 16 million fully analyzed proton-antiproton collisions, the CDF group made three separate searches for the top quark based on the different ways in which bottom quarks and W bosons (which also decay rapidly) are detected. Combining the results of the three searches yielded evidence for the top quark's production.
"It is possible that we are seeing a rare statistical fluctuation," says Galtieri, "but we have a good indication that the top quark may be there."
Physicists and engineers at LBL designed a sophisticated microchip for the Silicon Vertex Detector, an extremely high resolution instrument at the heart of the CDF array that enabled precise identification and tracking of bottom quarks. Galtieri's group also analyzed the CDF data to determine the mass of the top quark candidates. This analysis was based on a technique developed at LBL in the 1960's by the research group which was led by the late Nobel laureate Luis Alvarez.
"CDF's new evidence does not end the story but rather gives the first indications of a new field of research," says LBL physicist and CDF co-spokesperson William Carithers. "Many think that new phenomena are likely to show up at higher masses and the top quark is the most massive particle we know so far."
The CDF group expects that new data from another Tevatron experiment now underway will produce enough additional top quarks this year to confirm the particle's existence and to allow a more precise measurement of its mass. If the extremely large mass already reported is correct, it may help scientists better understand the process by which all objects acquire mass.
Members of LBL's CDF team included, in addition to Galtieri and Carithers, Willi Chinowski, Bob Ely, Kevin Einsweiler, Richard Kadel, Carl Haber, Young Kee Kim, Jeremy Lys, Manfried Paulini, Marjorie Shapiro, Hans Wenzel, and Weiming Yao. Participating students from the University of California at Berkeley included Bill Ashmanskas, Matt Austen, Mark Peters, and William Wester.
LBL and Fermilab are U.S. Department of Energy national laboratories. LBL is located in Berkeley, California and is managed by the University of California. Fermilab is located in Batavia, Illinois (about 30 miles west of Chicago) and is managed by Universities Research Association, Inc.