Search for the decay pi 0--> gamma X.

We present the results of a search for exotic bosons produced in the decay [pi][sup 0][r arrow][ital X][sub [gamma]]. A low energy [pi][sup [minus]] beam at TRIUMF was stopped in a liquid hydrogen target to produce the reactions [pi][sup [minus]][ital p][r arrow][ital n][gamma] and [pi][sup [minus]][ital p][r arrow][ital n][pi][sup 0],[pi][sup 0][r arrow][gamma][gamma]. The [gamma] rays from these reactions were detected in a sodium iodide crystal. From the energy distributions we extract a branching ratio limit for [Gamma]([pi][sup 0][r arrow][gamma][ital X])/[Gamma]([pi][sup 0][r arrow][gamma][gamma]) of (2--7)[times]10[sup [minus]3] for [ital X] masses of 25--100 MeV/[ital c][sup 2] (90% C.L.). In contrast with other experimental limits this branching ratio limit is valid for all reasonable [ital X] lifetimes ([gt]10[sup [minus]23] sec).