  Several models of dark matter motivate the concept of hidden sectors
consisting of SU(3)_C x SU(2)_L x U(1)_Y singlet fields. The interaction
between our and hidden matter could be transmitted by new abelian U'(1) gauge
bosons A' mixing with ordinary photons. If such A's with the mass in the
sub-GeV range exist, they would be produced through mixing with photons emitted
in two photon decays of \eta,\eta' neutral mesons generated by the high energy
proton beam in a neutrino target. The A's would then penetrate the downstream
shielding 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 an
excess 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 limits
on 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 magnitude
more restrictive than the previous bounds from the Crystal Barrel experiment.
