The 1995 Nobel Prize in Physics was awarded “for pioneering contributions to lepton physics,” with the award split jointly and evenly between Martin L. Perl “for the discovery of the tau lepton” and Frederick Reines “for the detection of the neutrino.”
The Science: Leptons
Throughout the twentieth centuries, physicists discovered much about the fundamental particles that compose matter in our universe.
The electron was discovered early on (theorized in 1874 and discovered in 1897), but it took a while longer for physicists to realize that there existed heavier particles that were similar the electron. These particles are collectively called leptons.
In addition to the electron there is a heavier lepton called the muon, which has a mass about 200 times heavier than the mass of the electron. The muon was discovered by Carl Anderson in 1936, while researching cosmic rays. With this discovery, it was said that the electron (and its related electron-neutrino) were the first “family” of leptons, while the muon (and its related muon-neutrino) were the second “family” of leptons.
The third “family” of leptons was discovered in a series of experiments at the Stanford Linear Accelerator Center (SLAC) between 1974 and 1977, with the discovery of the tau lepton (sometimes called just “tau”). This research was conducted by Martin L. Perl and his colleagues, and is the reason for the awarding of this prize.
The existence of this particle helped explain charge and parity violation (see: CP symmetry) and complete the Standard Model of particle physics.
The Science: Neutrinos
Neutrinos were proposed by Wolfgang Pauli in 1930, in trying to explain radioactive decay without violating conservation laws. His approach required the creation of very light, very fast-moving particles (later dubbed neutrinos by Enrico Fermi, who added refinements to the concept).
During the 1950’s, Frederick Reines and Clyde L. Cowan, Jr., were able to experimentally demonstrate the existence of the electron’s antineutrino. This is very difficult, because of how weakly neutrinos interact with atoms (when they bother to interact at all). Their original neutrino detector used only about half of a cubic meter of water, setting the stage for modern neutrino detectors that fill vast underground chambers with thousands of cubic meters of water to detect the neutrinos from cosmic rays that bombard our planet constantly.
Martin L. Perl
Martin L. Perl was born on June 24, 1927, in New York City. His parents immigrated to the United States from Russia as children, their families fleeing the poverty and antisemitism. His father founded a printing company and worked the family into the middle class. Martin went to college for chemical engineering, with delays for participating in the war in the U.S. Merchant Marines, he completed his undergraduate study and earned his degree in 1948 from Polytechnic University in Brooklyn. (He had skipped some years in school.)
Perl went to work for the General Electric Company, ultimately working in the Electron Tube Division and living in Schenectady, New York. Though a chemical engineer, he wanted to learn how electron vacuum tubes worked, so began taking courses at Union College, where he was encouraged by a professor to take up the study of physics.
Perl entered Columbia University’s doctoral physics program in 1950, where he studied under the 1944 Nobel Prize laureate, I.I. Rabi. Perl earned his doctorate in 1955, and then went on to the University of Michigan to conduct the research involving bubble chambers and spark chambers to study the physics of strong interactions. In 1963, Perl joined the staff at Stanford University working at the Stanford Linear Accelerator Center (SLAC), where he conducted the research that led to the discovery of the tau particle.
Martin Perl died September 30, 2014, in Palo Alto, California.
Frederick Reines was born on March 16, 1918, in Paterson, NJ. Like Perl, his parents were Jewish immigrants from Russia. Reines attended college at the Stevens Institute of Technology, studying engineering for a bachelor’s degree in 1939 and then obtaining his M.S. degree in mathematical physics in 1941. He then proceeded to New York University for his doctoral work, which he completed in 1944. During this time he began working at Los Alamos on the Manhattan Project, under the direction of Richard Feynman. During his time conducting research with Clyde Cowan, and the two of them were able to discover the electron antineutrino (for which this award is given to Reines alone, as Cowan was deceased by 1995). In 1959, he left Los Alamos for a position as the Professor and Head of the Physics Department at Case Institute of Technology in Cleveland, Ohio. He then went on to the University of California, Irvine, in 1966, having a number of administrative, research, and teaching positions within the physics department.
Frederick Reines died on August 26, 1998, in Orange, California.