By the early 1930's, physicists agreed that there were three fundamental constituents of matter: the electron, proton, and neutron. But many questions remained: What was the nature of the strong force holding the nucleus together? Why was the electron spectrum in b decay continuous? Did the antiparticles predicted by Dirac's theory exist? Each mystery was eventually resolved by the discovery of new particles. Already in 1932, Anderson discovered the positron, but not everyone immediately believed it was the antielectron. To explain the continuous b decay spectrum, Pauli proposed a new fundamental particle, the neutrino. The direct observation of neutrinos would occur only years later, but meanwhile Fermi developed the theory of b decay (1933-34), explicitly using neutrinos and proposing the weak interactions. Yukawa, during the same period, proposed a theory of the nuclear strong interaction and postulated a new particle, the pion, whose exchange between nucleons was responsible for the force.
Yukawa had predicted a mass of about 100 MeV, so the discovery in 1937 of such a particle in cosmic rays was immediately interpreted as the discovery of Yukawa's pion. It took ten years for physicists to realize that this particle did not interact strongly as it passed through matter. It was instead the muon. This discovery of a heavier clone of the electron was a stunning development. It was an early forerunner of a new generation of fundamental particles, particles with new "flavors."

The charged pion itself was found in cosmic rays in 1947, and a year later at an accelerator, marking the birth of particle physics as a field in its own right. Fermi and Yang suggested a revolutionary idea, that the pion might be a composite particle, consisting of a nucleon and an antinucleon. Though this proposal was wrong, fifteen years later the quark model explained mesons and baryons as composites.