The elucidation of the π → μv decay sequence left particle physics in a relatively simple state. Yukawa's particle had been found and the only unanticipated particle was the muon, of which I. I. Rabi is said to have remarked “Who ordered that?” The question remains unanswered. The cosmic-ray experiments of the next few years quickly and thoroughly destroyed the simplicity that had previously prevailed. The proliferation of new particles, many with several patterns of decay, produced great confusion. The primary source of confusion was whether each new decay mode represented a new particle or was simply an alternative decay for a previously observed particle. Continued experimentation with improved accuracy and statistics eventually resolved these ambiguities, but basic uncertainties remained. What was the nature of these particles? How were they related to the more familiar particles? The examination of these questions led to the development of the concepts of associated production, strangeness, and ultimately, parity violation and SU(3).

Remarkably, another meson seems to have been discovered before the pion. Working in the French Alps in 1943, Leprince-Ringuet and L'héritier took 10, 000 triggered pictures in a 75 cm × 15 cm × 10 cm cloud chamber placed inside a magnetic field of 2500 gauss (Ref. 3.1). This permitted careful measurements of the momenta of the charged tracks. One of the pictures showed an incident positive particle of about 500 MeV/c momentum produce a secondary of about 1 MeV/c.