Two teams confirm existence of elusive top quark

March 3, 1995

By Lynn Yarris,

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 D-Zero (D0) group--have independently observed the production and decay of what for nearly two decades had been the crucial missing piece in the scientific picture of matter.

"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 (PD) and a member of the D0 group, in a story reported in the March 1 San Francisco Chronicle.

"We were quite confident we had it last April but we were being conservative in our claims," said Lina Galtieri, a PD physicist who leads LBL participation in the CDF group, which announced the first evidence of the top quark last April (see Currents, April 29, 1994 ). At that time, the CDF group said it had a strong case for the top quark but lacked enough data to claim discovery.

Since then, the CDF and D0 groups have tripled the amount of data collected. Details of the experiments were presented during a March 2 symposium at Fermilab and papers have been submitted to Physical Review Letters. The top quark is reported to have 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.

Nearly a thousand scientists from around the world took part in the CDF and the D0 experiments; 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 of the top quark's signature decay products. The CDF's Hadronic End Calorimeter was also built at LBL.

The D0 group at LBL, which is led by Ron Madaras, was responsible for designing, building, testing, and commissioning two critical components of the D0 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.

Later this month, the CDF & D0 groups at LBL will present seminars on their findings.