Measurement of the $B_s^0\to J\mathrm{/}\psi K^{\ast 0}$ branching fraction and angular amplitudes

A search for the decay $B_s^0\to J\mathrm{/}\psi K^{\ast 0}$ with $K^{*0} \rightarrow K^-\pi^+$isperformedwith0.37fb$^{-1}$of$pp$collisionsat$\sqrt{s}$=7TeVcollectedbytheLHCbexperiment,findinga$\Bs \to J\psi K^-\pi^+$peakof$114 \pm 11$signalevents\mathrm{.}The$K^-\pi^+$ mass spectrum of the candidates in the $B_s^0$ peak is dominated by the $K^{\ast 0}$ contribution. Subtracting the non-resonant $K^{-}{\pi }^{+}$ component, the branching fraction of \BsJpsiKst is $(4.4_{-0.4}^{+0.5}\pm 0.8)\times {10}^{-5}$, where the first uncertainty is statistical and the second systematic. A fit to the angular distribution of the decay products yields the \Kst polarization fractions $f_L = 0.50 \pm 0.08 \pm 0.02$and$f_{||} = 0.19^{+0.10}_{-0.08} \pm 0.02$.