The new unstable cosmic-ray particles

A detailed description is given of the properties of the new unstable particles with masses between 400 m_e and 2200 m_e. Wilson cloud chamber experiments have provided definite evidence for the existence of a heavy neutral particle, the V₁⁰-particle, of mass 2190 m_e, which decays into a proton and a negative π-meson. A somewhat lighter neutral particle, known as a V₂⁰-particle, also exists, and possible decay schemes are discussed. The properties of the charged V-particles observed in Wilson cloud chambers are compared with the three main types of charged unstable particles found in photographic emulsions. The three types are the τ-meson, of mass 977 m_e, which decays into three charged π-mesons, the κ-particle, of mass about 1100 m_e, which decays into a μ-meson, and two or more neutral particles, and the χ-meson, of mass (900-1500) m_e, which decays into a charged π-meson and one neutral particle. The available data indicate that some of the S-particles found in cloud chambers are identical with the χ-particles, and that the charged V-particles probably consist of a mixture of κ- and χ-particles. The very slight evidence for the existence of charged particles with masses about 500 m_e is also described. A discussion is given of many of the possible decay schemes of the numerous particles, their frequencies of production and lifetimes.