PERL, MARTIN LEWIS

born June 24, 1927, New York, N.Y., U.S. American physicist who received the 1995 Nobel Prize for Physics for discovering a subatomic particle that he named the tau, a massive lepton with a negative charge. The tau, which he found in the mid-1970s, was the first evidence of a third "generation" of fundamental particles, the existence of which proved essential for completing the so-called standard model of particle physics. Perl was jointly awarded the Nobel Prize with physicist Frederick Reines (q.v.), who discovered another subatomic particle, the neutrino, in the 1950s. In 1948 Perl graduated from the Brooklyn Polytechnic Institute with a degree in chemical engineering. After working as a chemical engineer for two years, he studied nuclear physics at Columbia University (Ph.D., 1955). He was an instructor and associate professor at the University of Michigan (1955-63) before joining the faculty of Stanford University in 1963. In 1966 Perl was part of a research team that made an unsuccessful attempt to discover new charged leptons by colliding electrons at the Stanford Linear Accelerator Center (SLAC). A new particle accelerator that began operation at SLAC in the early 1970s had the capacity to reach high energy levels that were previously inaccessible. With this new machine, Perl recorded frontal collisions between electrons and their antiparticles, positrons. In a series of experiments conducted between 1974 and 1977, he found that the collisions formed heavy leptons, later called tau particles, that decay in less than a trillionth of a second into neutrinos and either an electron or a muon. He also discovered the antitau, which decays into neutrinos and either a positron or an antimuon.