Martin L. Perl

Martin L. Perl, born in 1927, American physicist and co-winner of the 1995 Nobel Prize in physics for his discovery of the tau lepton, a type of fundamental particle that makes up part of the atom. The 1995 prize was divided between Perl and American physicist Frederick Reines.

Perl, born in New York City, studied chemical engineering as an undergraduate at the Brooklyn Polytechnic Institute in New York. He received his Ph.D. degree in physics from Columbia University in 1950. Perl taught at the University of Michigan and then in 1964 joined the faculty of Stanford University, where he worked at the Stanford Linear Accelerator Center (SLAC).

The lepton is one of three groups of elementary particles, along with quarks and bosons. There are six known types of leptons, three that carry a negative electric charge and three with no charge. Of the negatively charged leptons, the electron was discovered in 1897, and the muon—basically a heavy electron—was discovered in the 1940s. Uncharged leptons are called neutrinos. Before Perl's discovery, theoretical physicists had not even predicted the existence of the tau, the third charged lepton. In 1973, Perl set out to study electrons and muons and to look for any differences between the two particles. By colliding subatomic particles, Perl was able to study both the particles and the products they produced after the collision. Occasionally, a collision resulted in the production of electrons and muons. Since Perl knew that muons were unstable particles that decayed, or broke down, into electrons and neutrinos, he theorized that only the decay of a more massive particle could result in the production of muons. In 1975, after analyzing tens of thousands of pieces of data, Perl announced his discovery of the tau lepton, a particle that is about 3500 times heavier than the electron.