  A search is presented for a hidden-sector boson, $\chi$, produced in the
decay $B^0 \rightarrow K^* (892)^0 \chi$, with $K^* (892)^0 \rightarrow K^+
\pi^-$ and $\chi \rightarrow \mu^+ \mu^-$ . The search is performed using a
$pp$-collision data sample collected at $\sqrt{s}=7$ and 8 TeV with the LHCb
detector, corresponding to integrated luminosities of 1 and 2 fb$^{-1}$
respectively. No significant signal is observed in the mass range $214 \le
m_\chi \le 4350$ MeV, and upper limits are placed on the branching fraction
product $\mathcal{B}(B^0 \rightarrow K^* (892)^0 \chi) \times \mathcal{B}(\chi
\rightarrow \mu^+ \mu^- )$ as a function of the mass and lifetime of the $\chi$
boson. These limits place the most stringent constraints to date on many
theories that predict the existence of additional low-mass dark bosons.
