Kaon Decays to Pions

Abstract

The International Conference on Cosmic Radiation held at Bagneres de Bigorre in July 1953 marked a “coming of age” for strange particle physics. The data presented by many groups, using a variety of experimental techniques, showed a remarkable degree of consistency. We were apparently faced with the discovery of many new unstable particles, produced in or near the detector used, through strong-interaction processes initiated by the incident cosmic ray particles. Cloud chamber observations had already established V⁰-events, especially those due to the (π^-p) decay of the neutral strange baryon now known as Λ(1115), and some V^±-events. Triggered cloud chambers at mountain stations had shown evidence of positive particles which came to rest in internal plates and decayed to give an energetic μ⁺-particle (or less often, an energetic π⁺ meson). Some of the clearest and most accurate data in this period was that provided by the study of events recorded in nuclear emulsions which had been exposed to the cosmic radiation at mountain altitudes. The most striking emulsion events reported at the conference were those termed τ⁺, where the particle came to rest in the emulsion and decayed to give three coplanar pions (π⁺π⁺π^-) with total momentum zero and a total kinetic energy of 75±2 MeV, corresponding to a mass m(τ⁺) of 494 ± 2 MeV, quite close to the value known for it today.