Meson Reactions in Hydrogen and Deuterium

In June, 1947, at the Shelter Island Conference on Theoretical Physics, the contradiction between the large production cross section for mesons in the high atmosphere and their subsequent extremely weak nuclear interaction at low altitudes were subjected to a careful arid searching scrutiny. Conversi, Pancini, and Piccioni had already performed their classic experiment on the absorption of slow negative mesons in matter of low atomic number and Fermi, Teller, and Weisskopf had shown that the observed decay of the negative mesons implied a meson interaction with nuclei weaker by a factor of 10 or more than that previously assumed. Many suggestions were offered to explain the apparent lack of reversibility between emission and absorption of mesons and the two-meson hypothesis put forward by the author as a way out of the difficulties seemed but another "shot in the dark". However, before the Shelter Island meeting was many weeks old, the first two-meson photographs obtained by Powell and his collaborators reached the U. S. and provided the initial evidence in favor of the two-meson theory. Since then many experiments have confirmed the idea that the heavier or π-mesons are strongly coupled to nucleons whereas the lighter or μ-mesons are the decay products of π-mesons and experience weak interactions with nucleons. Further, we shall not enter into a discussion of these experiments here but it is useful to point out that all these experiments imply that π-mesons should react vigorously with the nuclei of hydrogen and deuterium whereas μ-meson reactions with these two lightest nuclei should be quite unobservable. Hence, when we speak of meson reactions in hydrogen and deuterium, we mean π-meson reactions.