"The first intimations that β decay is but one manifestation of some deeper fundamental interaction came during the 1940s from experiments which led to the discovery of the . ...A third charged , the tau, and three neutral neutrinos bring the number of family members to six. In addition there are six corresponding antiparticles. It appears that in any interaction a lepton can be created (or can disappear) only together with an antilepton. This empirical rule of 'lepton conservation'... implies... that it is an antineutrino that accompanies the electron in β decay. ...When the decay or capture of a muon was treated in the same way as β decay in Fermi's theory, the s... appeared remarkably similar. ...The agreement between the coupling constants for β decay, muon decay and muon capture led to the idea of a 'universal Fermi interaction' and... experiments began to reveal more and more new particles with similar weak interactions."