The fate of the mesotron. The Rome experiment on the nuclear absorption of hard cosmic rays

In the 1938--1946 period, cosmic ray studies, nuclear physics, and quantum field theory merged in the cluster of researches related to the spontaneous decay of the particle then called mesotron, or meson. For the first time, an elementary particle was observed to be spontaneously unstable. The conceptual framework and the material apparatus far the study of this new phenomenon had to be created and stabilized. Throughout the last part of World War II and its immediate aftermath, a team of young Italian physicists pursued and achieved the "direct" observation of mesotron decays. They worked within the Italian cosmic-ray tradition while successfully innovating upon its instrumentation and experimental methods. The acquired control of mesotron decay, together with the then current understanding of the nature of mesotrons, directed them to the investigation of another new phenomenon, the capture of mesotrons by atomic nuclei. The outcome of their experiment was unexpected and had far-reaching consequences.