1995 Nobel Prize in Physics

The prize was awarded jointly to:

• Perl, Martin L., U.S.A., Stanford Linear Accelerator Center, Stanford University, Stanford, CA

"for pioneering experimental contributions to lepton physics, specifically for the discovery of the tau lepton";

and

• Reines, Frederick , U.S.A., University of California at Irvine, Irvine, CA

"for pioneering experimental contributions to lepton physics, specifically for the detection of the neutrino".

Tau: The 3rd Electron-like Particle

In 1975, Martin Perl (SLAC) scanned the 1973-1974 SPEAR experimental data, searching for a particularly unusual type of event. What if, he supposed, sometimes an electron and positron annihilate, and the detector records only one electron-type track, and one muon-type track?

These theoretically-predicted events were found, and at rates that could only be explained by postulating another new particle type, one just like the electron but 3,000 times more massive. (The muon, too, is just like the electron, but 200 times more massive and no one -- yet -- understands why there are three electron-like particles.)

Martin Perl Receives The Nobel Prize (photo by Joseph Perl)

When a pair of taus, one tau plus and one tau minus, is produced, each decays rapidly. In addition to unseen neutrinos, sometimes one produces a muon, and the other an electron.

Computer reconstruction of an event at the SPEAR Mark I detector showing only one electron track and one muon track. Note that the tracks are not "back-to-back". This tells us some unseen neutrinos carried off momentum