Meson Masses and Energetics of Meson Decay

A series of measurements of the π- and μ-meson masses and their decay energies is reviewed. Recent mass measurements made at the University of California Radiation Laboratory have employed an improved method in which the ratio of the π-meson mass to that of the proton and the ratio of the μ-meson mass to that of the π-meson were determined by measuring the ratios of ranges and ratios of momenta for particles of equal velocities. The absolute decay momentum of the μ-meson was also determined in the same series of experiments. Other experiments at the Radiation Laboratory in which quite different methods were used to obtain the π-meson mass and the decay energy of the μ-meson are also discussed. The measurements are consistent with a π-meson mass of 276 electron masses and a μ-meson mass of 210 electron masses. In addition, no contradiction is found for the assumptions that the π − μ decay is accompanied by a neutrino and the μ − e decay is accompanied by two neutrinos.