  A search is presented for hidden-sector bosons, $\chi$, produced in the decay
${B^0\!\to K^*(892)^0\chi}$, with $K^*(892)^0\!\to K^{+}\pi^{-}$ and
$\chi\!\to\mu^+\mu^-$. 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
$\mathcal{B}(B^0\!\to K^*(892)^0\chi)\times\mathcal{B}(\chi\!\to\mu^+\mu^-)$ 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.
