A search is presented for hiddensector 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 lowmass bosons.
Feynman diagram for the decay $ B^0 \!\to K^{*0}\chi$ , with $\chi \! \to \mu^+\mu^$. 
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Invariant mass spectrum with fit overlaid for all prompt $ B^0 \!\to K^{*0}\mu^+\mu^$ candidates with $1.1 < m^2(\mu^+\mu^) < 6.0\mathrm{\,GeV} ^2$. 
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Distribution of $ m(\mu^+\mu^)$ in the (black) prompt and (red) displaced regions. The shaded bands denote regions where no search is performed due to (possible) resonance contributions. The $ {J\mskip 3mu/\mskip 2mu\psi\mskip 2mu}$ , $\psi(2S)$ and $\psi(3770)$ peaks are suppressed to better display the search region. 
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Upper limits at 95% CL for (left axis) $\mathcal{B}( B^0 \!\to K^{*0}\chi (\mu^+\mu^))/\mathcal{B}( B^0 \!\to K^{*0}\mu^+\mu^ )$, with $ B^0 \!\to K^{*0}\mu^+\mu^$ in $1.1 < m^2(\mu^+\mu^) < 6.0\mathrm{\,GeV} ^2$, and (right axis) $\mathcal{B}( B^0 \!\to K^{*0}\chi (\mu^+\mu^))$. The sparseness of the data leads to rapid fluctuations in the limits. Excluding the region near $2m(\mu)$, the relative limits for $\tau < 10\,ps $ are between 0.0050.05 and all relative limits for $\tau \leq 1000\,ps $ are less than one. 
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Exclusion regions at 95% CL: (left) constraints on the axion model of Ref. [20]; (right) constraints on the inflaton model of Ref. [52]. The regions excluded by the theory [52] and by the CHARM experiment [53] are also shown. 
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(left) Integral values for 1s, 2s and 2c relative to the value for 1c (see text for details). The dashed lines show the values in the absence of inefficiency. (right) Ratio of the efficiency for an unpolarized spinone boson to that of a spinzero boson. 
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Upper limits at 95% CL for (left axis) $\mathcal{B}( B^0 \!\to K^{*0}\chi (\mu^+\mu^))/\mathcal{B}( B^0 \!\to K^{*0}\mu^+\mu^ )$, with $ B^0 \!\to K^{*0}\mu^+\mu^$ in $1.1 < m^2(\mu^+\mu^) < 6.0\mathrm{\,GeV} ^2$, and (right axis) $\mathcal{B}( B^0 \!\to K^{*0}\chi (\mu^+\mu^))$. Same as Fig. 4 in the Letter but including the $\tau = 0$ and 1\,ps limits. 
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Upper limits at 95% CL for (top) $\mathcal{B}( B^0 \!\to K^{*0}\chi (\mu^+\mu^))/\mathcal{B}( B^0 \!\to K^{*0}\mu^+\mu^ )$, with $ B^0 \!\to K^{*0}\mu^+\mu^$ in $1.1 < m^2(\mu^+\mu^) < 6.0\mathrm{\,GeV} ^2$, (middle) $\mathcal{B}( B^0 \!\to K^{*0}\chi (\mu^+\mu^))$, and (bottom) both relative and absolute limits. The $\omega$ and $\phi$ resonance regions are only excluded in the prompt region. A utility is provided to obtain these limits for any $( m(\chi) ,\tau(\chi) )$ on the CERN Document Server. 
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Animated gif made out of all figures. 
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Created on 15 February 2019.Citation count from INSPIRE on 21 February 2019.