At the H. H. Wills Physical Laboratory of the University of Bristol, the group led by Cecil Powell develops from the late 1930s onward the technique of special photographic emulsions for recording the trajectories of cosmic-ray particles with great precision. The Brazilian physicist César Lattes, arriving in Bristol in 1946 to work with his former teacher Occhialini, improves the sensitivity of these emulsions by adding boron. On March 7, 1947, two of the group's microscopy technicians, Marietta Kurz and Irene Roberts, examine plates exposed at altitude and detect an anomalous event: a meson that stops and, from that same point, emits a second, lower-energy meson. Powell and a young graduate, Hugh Muirhead, immediately recognize this as two distinct particles in a decay chain, not a single one. More similar events appear within days. The finding confirms the existence of the heavy meson predicted in 1935 by Hideki Yukawa as the carrier of the strong nuclear force — the pion — and reveals that it decays into a second, lighter meson, the muon. Lattes, Muirhead, Occhialini, and Powell publish the result in "Processes Involving Charged Mesons" (Nature, May 24, 1947); months later, Lattes travels to Chacaltaya (Bolivia, 5,200 m) and to the Pic du Midi (France) to expose more plates at altitude and confirm the finding with greater statistics, publishing together with Occhialini and Powell a second paper in October 1947. In 1948, now at Berkeley together with Eugene Gardner, Lattes succeeds in artificially producing pions in the cyclotron, closing the verification cycle. The discovery resolves a central enigma of nuclear physics from the 1930s-40s — the nature of the force holding the atomic nucleus together — and opens particle physics as an independent experimental discipline. Cecil Powell receives the 1950 Nobel Prize in Physics "for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made with this method"; Lattes, despite being the principal investigator and first author of the Nature paper, is not included in that prize.