Evidence grows for charged heavy lepton at 1.8–2.0 GeV Available to Purchase

Early in 1975 Martin Perl and his collaborators at SLAC observed anomalous production of an electron–muon pair in electron–positron annihilation. At the time (PHYSICS TOOAY, October l975, page 17) the group thought they could be observing a heavy lepton, a heavy meson or an elementary boson. By now, the returns from four additional detectors are in. The detector groups reported their results at the International Symposium on Lepton and Photon Interactions at High Energies held in Hamburg late in August. All the groups believe the most likely explanation for the anomalous events is a charged heavy lepton, now called the “tau,” a pointlike particle with mass 1.8–2.0 GeV. The heavy lepton would join its lighter siblings—the electron, muon, electron neutrino and muon neutrino. Presumably the τ would also have its own neutrino. The particle is called τ because it appears to be the third charged lepton to be found, and the Greek word for third begins with τ.