  A search for a long-lived scalar particle $\chi$ is performed, looking for
the decay ${B^+ \to K^+ \chi}$ with ${\chi \to \mu^+\mu^-}$ in $pp$ collision
data corresponding to an integrated luminosity of $3\, {\rm fb}^{-1}$,
collected by the LHCb experiment at centre-of-mass energies of $\sqrt{s}=7$ and
8$\,$TeV. This new scalar particle, predicted by Hidden Sector models, is
assumed to have a narrow width. The signal would manifest itself as an excess
in the dimuon invariant mass distribution over the Standard Model background.
No significant excess is observed in the accessible ranges of mass ${250 <
m(\chi) < 4700\,\rm MeV/c^2}$ and lifetime ${0.1 < \tau(\chi) < 1000\,\rm ps}$.
Upper limits on the branching fraction $\mathcal{B}(B^+ \to K^+ \chi
(\mu^+\mu^-))$ at 95% confidence level are set as a function of $m(\chi)$ and
$\tau(\chi)$, varying between $2\times10^{-10}$ and $10^{-7}$. These are the
most stringent limits to date. The limits are interpreted in the context of a
model with a light inflaton particle.
