A search is presented for hidden-sector bosons, $chi$, produced in the decay ${B^0!to K^*(892)^0chi}$, with $K^*(892)^0!to K^{+}pi^{-}$ and $chi!tomu^+mu^-$. The search is performed using $pp$-collision data corresponding to 3.0 fb$^{-1}$ collected with the LHCb detector. No significant signal is observed in the accessible mass range $214 leq m({chi}) leq 4350$ MeV, and upper limits are placed on the branching fraction product $mathcal{B}(B^0!to K^*(892)^0chi)timesmathcal{B}(chi!tomu^+mu^-)$ as a function of the mass and lifetime of the $chi$ boson. These limits are of the order of $10^{-9}$ for $chi$ lifetimes less than 100 ps over most of the $m(chi)$ range, and place the most stringent constraints to date on many theories that predict the existence of additional low-mass bosons.
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