Evidence for the decay $B_s^0\to {\overline{K}}^{\ast 0}{\mu }^{+}{\mu }^{-}$

A search for the decay $B_s^0\to {\overline{K}}^{\ast 0}{\mu }^{+}{\mu }^{-}$ is presented using data sets corresponding to 1.0, 2.0 and 1.6 fb${}^{-1}$ of integrated luminosity collected during $pp$ collisions with the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV, respectively. An excess is found over the background-only hypothesis with a significance of 3.4 standard deviations. The branching fraction of the $B_s^0\to {\overline{K}}^{\ast 0}{\mu }^{+}{\mu }^{-}$ decay is determined to be $B(B_s^0\to {\overline{K}}^{\ast 0}{\mu }^{+}{\mu }^{-})=[2.9\pm 1.0(stat)\pm 0.2(syst)\pm 0.3(norm\left)\right]\times {10}^{-8}$, where the first and second uncertainties are statistical and systematic, respectively. The third uncertainty is due to limited knowledge of external parameters used to normalise the branching fraction measurement.