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Several models of dark matter motivate the concept of hidden sectorsconsisting of SU(3)_C x SU(2)_L x U(1)_Y singlet fields. The interactionbetween our and hidden matter could be transmitted by new abelian U'(1) gaugebosons A' mixing with ordinary photons. If such A's with the mass in thesub-GeV range exist, they would be produced through mixing with photons emittedin two photon decays of \eta,\eta' neutral mesons generated by the high energyproton beam in a neutrino target. The A's would then penetrate the downstreamshielding and be observed in a neutrino detector via their A'-> e+e- decays.Using bounds from the CHARM neutrino experiment at CERN that searched for anexcess of e+e- pairs from heavy neutrino decays, the area excluding the \gamma- A' mixing range 10^{-7} < \epsilon < 10^{-4} for the A' mass region 1 < M_A'<500 MeV is derived. The obtained results are also used to constrain models,where a new gauge boson X interacts with quarks and leptons. New upper limitson the branching ratio as small as Br(\eta -> \gamma X) < 10^{-14} and Br(\eta'-> \gamma X) < 10^{-12} are obtained, which are several orders of magnitudemore restrictive than the previous bounds from the Crystal Barrel experiment.

Constraints on sub-GeV hidden sector gauge bosons from a search for heavy neutrino decays