A shoot to the heaviest lepton (1975) was expected just after the discovery a year earlier of the J/Y particle, i.e. of the first charming meson. The Nobel Prize was awarded 20 years after this "shoot"*.
t lepton is heavier than the J/Y meson, it is also very unstable - it decays before being detected to electrons and other light particles.
*) 1995 Nobel 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 dection of the neutrino".