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| Program > Experiments |
| Programme > L'expérimentation |
| Daniela
Monaldi
(University of Toronto) The Conversi-Pancini-Piccioni Experiment and the Beginning of Modern Particle Physics |
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At the beginning of the 1940's, the particles called mesotrons, or mesons, held the promise of a unified theory of nuclear phenomena. Mesotrons had been found to constitute the most penetrating component of cosmic rays. They were heavier than electrons and lighter than protons, just as the hypothetical quanta of a theory formulated by Yukawa. Moreover, the mesotrons decayed emitting electrons, and Yukawa's nuclear quanta were expected to be responsible for b-radioactivity by disintegrating into electrons and undetectable neutrinos. Were not the mesotrons the key to the atomic nucleus, to its binding as well as to its radioactivity? From 1941 and through the difficult years of World War II, three young Italian physicists, Oreste Piccioni, Marcello Conversi, and Ettore Pancini, carried on a series of observations of mesotrons stopped in matter, which seemed at the beginning to support Yukawa’s predictions. They were able to achieve a high level of precision and stability by innovating upon instruments and techniques of the Italian cosmic ray tradition. At the end of 1946, they reported that the rates of absorption of mesotrons in light materials were in catastrophic disagreement with the theory. If the mesotrons were not the nuclear quanta, what were they? And what new kind of phenomenon was their b-like decay? The physics community would have to sort through a bewildering diversification before prospects of unification could reappear, on a more remote level.
University
of Toronto
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