Conflicting evidence for K‐meson decay

The long‐lived K‐meson may decay according to predictions after all. Last year, experiments at the Berkeley Bevatron had failed to find evidence of the reaction $KL0←μ++μ−.$ Accepted theoretical estimates of the ratio of this decay to all other decays (the branching ratio) give a lower bound of about $6×10−9,$ but the Berkeley group established an upper limit of about $1.8×10−9,$ with a 90% confidence level, causing some theorists to question such basic ideas as the nature of muon interactions, the idea that CP is conserved in the decay and even the principles of unitarity (probability conservation) and quantum electrodynamics (see PHYSICS TODAY, July 1971, page 13).