This book reports on a new result from the K_L→π⁰νν search at the J-PARC KOTO experiment, which sets an upper limit of 3×10^-9 for the branching fraction of the decay at the 90% confidence level, improving the previous best limit by an order of magnitude. To explain the matter–antimatter asymmetry in the universe, still unknown new physics beyond the standard model (SM) that breaks CP symmetry is necessary. The rare decay of a long-lived neutral K meson, K_L→π⁰νν, is a...