Along with all the discoveries of mesons and baryons was the critical detection of the neutrino by Cowan and Reines in 1956, demonstrating the feasibility of detecting neutrino-induced interactions. Nishijima suggested in 1957 that there were two kinds of "lepton charge" (now called flavors), one for e and ne and one for m and nm. In 1963, using a beam of neutrinos from pion and K decays, Lederman, Schwartz, and Steinberger demonstrated the existence of the muon neutrino, distinct from the electron neutrino.
	The next year, several physicists suggested that if the m and nm belonged to a second generation of leptons, then perhaps there was a second generation of quarks. If the d and u quarks were the first generation quarks, then the s quark belonged to the second generation, but a new heavier quark with charge 2/3 would also be needed. Bjorken and Glashow called this fourth quark the "charm" quark. However, since the charm quark had not been found, the concept of flavors (generations) was not widely accepted. Six years later Glashow, Iliopoulos, and Maiani (GIM) found a much more compelling argument for the charm quark: it allowed a theory of weak interactions with flavor-conserving, but not flavor-changing, neutral currents.
	Back in the 1950's, there were other developments in understanding the weak interactions. Lee and Yang proposed that parity might not be conserved in weak processes, and almost immediately experimental searches uncovered violations of parity in weak decay processes. In 1957 Landau and separately Lee, Oehme, and Yang suggested that the combined symmetry of CP (charge conjugation and parity) might still remain valid; Pais and Treiman noted that decays of neutral K's could test this idea. Also proposed (Schwinger, Bludman, and Glashow) was the existence of a new particle, the W boson, that mediated the weak interactions.