Colliding proton and antiproton produces top quarks.

Thanks to two recent discoveries at Fermi National Accelerator Laboratory, the third family of fundamental constituents of matter has been completed. In 1995, two experiments discovered the "top" quark, the last undiscovered quark of the six predicted to exist by current theory. Quarks are the smallest particles of matter; the top quark is the heaviest elementary particle known. Scientists worldwide had been seeking the top quark since 1977, when the third family's "bottom" quark was discovered at Fermilab. An experiment in 2000 found the elusive tau neutrino, completing the third generation of leptons (weakly interacting particles). Scientists were convinced it existed, but no one before had directly observed this near-massless particle, which barely interacts with surrounding matter. The team used elaborate equipment-including very strong magnets to remove all charged particles and 15 meters of shielding to remove all else but neutrinos-and spent three years looking for the neutrino's tracks, found in just four of 6 million potential interactions. The three families of quarks and leptons, together with forces that govern their interactions, make up the Standard Model of particle physics. The only missing piece now is the Higgs boson, the expected source of mass.

Scientific Impact: These discoveries required more than 30 years of technological advances and confirmed the value of equipment such as the Tevatron, the world?s highest-energy particle accelerator. Discovery of the tau neutrino also opened a door to studies of the connections among neutrinos and whether neutrinos have mass.

Social Impact: This confirmation of existing theory contributed to science education. In addition, although basic research is by definition a search for new knowledge without regard to its practical implications, such work often contributes to technologies with commercial value; examples include computers, lasers, and cancer treatments.

Reference: "Observation of the Top Quark," F. Abe et al., Phys. Rev. Lett. 74: 2626 (1995).

"Observation of the Top Quark," S. Abachi et al., Phys. Rev. Lett. 74: 2632 (1995).

URL: http://www.fnal.gov/pub/inquiring/physics/discoveries/top_quark_background/top95_background.html

Technical Contact: Dr. William Carithers, Jr., wccarithers@lbl.gov
Prof. Paul Grannis, pgrannis@sunysb.edu

Press Contact: Jeff Sherwood, DOE Office of Public Affairs, 202-586-5806

SC-Funding Office: Office of High Energy and Nuclear Physics