| Summary: | <p>First observation of the decay <i>B</i><sub><i>s</i></sub><sup>0</sup> → <i>K</i><sub>S</sub><sup>0</sup><i>K</i><sup>∗</sup>(892)<sup>0</sup> at LHCb</p> <p>A search for <i>B</i><sub>(<i>s</i>)</sub><sup>0</sup> → <i>K</i><sub>S</sub><sup>0</sup><i>K</i><sup>∗</sup>(892)<sup>0</sup> decays is performed using <i>pp</i> collision data, corresponding to an integrated luminosity of 1.0 fb<sup>−1</sup>, collected with the LHCb detector at a centre-of-mass energy of 7 TeV. The <i>B</i><sub><i>s</i></sub><sup>0</sup> → <i>K</i><sub>S</sub><sup>0</sup><i>K</i><sup>∗</sup>(892)<sup>0</sup> decay is observed for the first time, with a significance of 7.1 standard deviations. The branching fraction is measured to be<br/> 𝓑(<i>B</i><sub><i>s</i></sub><sup>0</sup> → <i>K̅</i><sup>0</sup><i>K</i><sup>∗</sup>(892)<sup>0</sup>) + 𝓑(<i>B</i><sub><i>s</i></sub><sup>0</sup> → <i>K</i><sup>0</sup><i>K̅</i><sup>∗</sup>(892)<sup>0</sup>)=(16.4 ± 3.4 ± 2.3) × 10<sup>−6</sup>,<br/> where the first uncertainty is statistical and the second is systematic. No evidence is found for the decay <i>B</i><sup>0</sup> → <i>K</i><sub><i>S</i></sub><sup>0</sup><i>K</i><sup>∗</sup>(892)<sup>0</sup> and an upper limit is set on the branching fraction, 𝓑(<i>B</i><sup>0</sup> → <i>K̅</i><sup>0</sup><i>K</i><sup>∗</sup>(892)<sup>0</sup>) + 𝓑(<i>B</i><sup>0</sup> → <i>K</i><sup>0</sup><i>K̅</i><sup>∗</sup>(892)<sup>0</sup>) < 0.96 × 10<sup>−6</sup>, at 90 % confidence level. All results are consistent with Standard Model predictions.</p>
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