Discovering the
Top Quark

After nearly two decades of searching, scientists have declared that the top quark, the last of the six quarks predicted by the Standard Model of particle physics and one of the fundamental building blocks of matter, has been discovered. Both of the main experimental groups at Fermilab's Tevatron collider -- the Collider Detector Facility (CDF) and the DZero group -- have independently observed the production and decay of what had been the crucial missing piece in the scientific picture of matter.

Electronic signatures produced by collisions of protons and antiprotons in the Tevatron accelerator at Fermilab provided evidence that the elusive subatomic particle known as top quark has been found.

"This is one of the ten most important accomplishments in this century in this field," said Tom Trippe, a physicist in LBL's Physics Division and a member of the DZero group.

"We were quite confident we had it last April, but we were being conservative in our claims," said Lina Galtieri, who leads LBL participation in the CDF group which announced the first evidence of the top quark last April. At that time, the CDF group said it had strong case for the top quark but lacked enough data to claim discovery. Since then, the CDF and DZero groups have tripled the amount of data collected.

Nearly a thousand scientists from around the world took part in the CDF and the DZero experiments, and LBL researchers made vital contributions to both collaborations. 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 detector system that enabled precise identification and tracking of bottom quarks, one the top quark's signature decay products. The CDF's hadronic end calorimeter was also built at LBL.

The DZero group at LBL, which is led by Ron Madaras, was responsible for designing, building, testing, and commissioning two critical components of the DZero detector array: the Electromagnetic End Calorimeter which identifies and measures the energies of electrons; and the Vertex Detector which detects the tracks left by charged particles.

In announcing the discovery, project spokespersons reported that the top quark has about the same mass as an atom of gold or about 200 times the mass of a proton, making it by far the heaviest elementary particle ever observed.

Top quarks exist only for an instant before decaying into a bottom quark and a W boson, which means those created at the birth of the universe are long gone. Even the Tevatron, the most powerful collider in the world, has difficulty creating top quarks. It is estimated that one pair of top quarks is produced for every 10 billion proton-antiproton collisions. According to the Standard Model, however, the existence of the top quark at creation was necessary along with the other five types of quarks. The only types that have since survived in nature are the up and the down quarks, the pair that make up the protons and neutrons in the nuclei of all the known atoms in the universe.

-- Lynn Yarris