Search for the decay ${\mathrm{\pi }}^0$→γ+X

A search for the decay ${\mathrm{\pi }}^0$→γX, where X is any long-lived weakly interacting neutral vector particle with mass smaller than the neutral pion pass, was performed using neutral pions tagged by ${\mathit{K}}^{+}$→${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^0$. A 90%-C.L. upper limit for the branching ratio of the two-body decay B(${\mathrm{\pi }}^0$→γX)<5×${10}^{\mathrm{-}4}$ is set. Limits are also set for three-body decays ${\mathrm{\pi }}^0$→γXX’.