We report new experimental limits on the π^0 -> γ X decay of the neutral pion into a photon and a light boson X followed by the decay X -> e+e-. If this process exist, one would expect a flux of high energy X's produced from π^0's generated by the proton beam in a neutrino target. The X's would then penetrate the downstream shielding and be observed in a neutrino detector via their decays. Using bounds from the NOMAD and PS191 neutrino experiments at CERN that searched for an excess of e+e-

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... s from heavy neutrino decays, stringent limits on the branching ratio as small as Br(π^0 -> γ X) < 10^-15 are obtained. These limits are several orders of magnitude smaller than the previous experimental and cosmological bounds. The obtained results are used to constrain models, where the X interacts with quarks and leptons, or it is a new vector boson mixing with photons, that transmits interaction between our world and hidden sectors consisting of SU(3)_C x SU(2)_L x U(1)_Y singlet fields.