A search for $B_{(s)}^{0} \to K_{S}^{0} K^{*}(892)^{0}$ decays is performed using $pp$ collision data, corresponding to an integrated luminosity of $1.0 \text{fb}^{1}$, collected with the LHCb detector at a centreofmass energy of $7 \text{TeV}$. The $B_{s}^{0} \to K_{S}^{0} K^{*}(892)^{0}$ decay is observed for the first time, with a significance of 7.1 standard deviations. The branching fraction is measured to be \begin{equation*} \mathcal{B}(B_{s}^{0} \to \bar{K}^{0} K^{*}(892)^{0}) + \mathcal{B}(B_{s}^{0} \to K^{0} \bar{K}^{*}(892)^{0}) = (16.4 \pm 3.4 \pm 2.3) \times 10^{6},\\ \end{equation*} where the first uncertainty is statistical and the second is systematic. No evidence is found for the decay $B^{0} \to K_{S}^{0} K^{*}(892)^{0}$ and an upper limit is set on the branching fraction, $\mathcal{B}(B^{0} \to \bar{K}^{0} K^{*}(892)^{0}) + \mathcal{B}(B^{0} \to K^{0} \bar{K}^{*}(892)^{0}) < 0.96 \ \times 10^{6}, $ at $90 \% $ confidence level. All results are consistent with Standard Model predictions.
Distribution of (left) $ K ^0_{\rm\scriptscriptstyle S} K ^\pm \pi ^\mp $ mass and (right) $ K ^\pm$ {} $\pi ^\mp$ mass for signal candidates with fit results overlaid for (top) downstream and (bottom) long categories. The data are shown as black points with error bars. The overall fit is represented by the solid black line. The $ B ^0$ and $ B ^0_ s $ signal components are the black shortdashed and dotted lines respectively, while the nonresonant components are the magenta shortdashed and dotted lines. The partially reconstructed backgrounds are the red tripledotted line ($ B \rightarrow Dh$) and the blue tripledotted line ( $ B ^0_ s \rightarrow K ^{*0} \overline{K}^{*0}$ ). The combinatorial background is the green longdash dotted line. 
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Distribution of $ K ^0_{\rm\scriptscriptstyle S} \pi ^+ \pi ^ $ mass for signal candidates with fit results overlaid for (left) downstream and (right) long categories. The data are described by the black points with error bars. The overall fit is represented by the solid black line. The $ B ^0$ and $ B ^0_ s $ signal components are the black shortdashed and dotted lines. The misidentified $ B ^0_ s $ decay is the black dashed line, respectively. The partially reconstructed backgrounds are the red tripledotted line ($ B \rightarrow Dh$), the blue tripledotted line ($ B ^0 \rightarrow K ^0_{\rm\scriptscriptstyle S} \pi ^+ \pi ^ X$), the violet dash singledotted line ($ B ^0 \rightarrow \eta' K ^0_{\rm\scriptscriptstyle S} $) and the pink shortdash single dotted line ($ B ^0 \rightarrow K ^0_{\rm\scriptscriptstyle S} \pi ^+ \pi ^ \gamma$). The combinatorial background is the green longdash dotted line. Some of the contributions are small in the figures. 
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Animated gif made out of all figures. 
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Signal yields obtained from the fits to $ K ^0_{\rm\scriptscriptstyle S} K ^\pm \pi ^\mp $ and $ K ^0_{\rm\scriptscriptstyle S} \pi ^+ \pi ^ $ mass distributions and corresponding efficiencies. Only statistical contributions to the uncertainty are reported. 
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Systematic uncertainties on the relative branching fraction measurement for the two $ K ^0_{\rm\scriptscriptstyle S}$ categories. The uncertainties are quoted as fractional contributions of the relative branching fraction and the total is the sum in quadrature of all contributions. 
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Created on 11 July 2020.