Martin Perl, a SLAC physicist with initial training in chemical engineering, has for years been convinced that there is no underlying theoretical reason to assume only two families of leptons exist — electron and muon, each with its associated neutrino. When SLAC's SPEAR electron-positron collider comes online, Perl sees the practical opportunity to search for a heavier third lepton, joining his experimental group with Burton Richter's and a Lawrence Berkeley Laboratory team to build the detector known as Mark I. Analyzing electron-positron collision data, the team finds in 1975 a set of anomalous events: pairs of particles with an electron and a muon of opposite sign, with no other visible particle appearing to balance the event's energy and momentum. These "e-mu" events could not be explained by any particle or process known at the time. Perl and his team publish the finding in "Evidence for Anomalous Lepton Production in e+e- Annihilation" (Physical Review Letters, 1975), proposing that the events are explained by the production of a pair of heavy, previously unknown leptons, each decaying into a lighter lepton (electron or muon) and two invisible neutrinos — hence why energy and momentum appeared not to be conserved in the observed event. The initial proposal is received with skepticism: for almost two years, other groups attempt to explain the events via alternative mechanisms, such as production of charm-quark D-meson pairs. Independent confirmations at DORIS in Hamburg (PLUTO and DASP experiments) in 1976-1977 finally dispel the doubts. In 1977, now convinced it was a genuine lepton, Perl names it "tau" (τ), from the Greek letter for "third" (triton), as the third charged lepton discovered. The finding is the first experimental evidence of a third generation of elementary particles — discovered the same year, independently, as Leon Lederman confirms at Fermilab the existence of the upsilon particle and the bottom quark (see separate entry herb-lederman-upsilon-bottom-quark-1977) — together with that finding completing the first solid proof that the pattern of particle generations repeats a third time. For this work, Perl receives the 1995 Nobel Prize in Physics, twenty years after the discovery.