Evidence for the decay $B_s^0\to \phi {\eta }^{\mathrm{\prime }}$

Using a dataset corresponding to an integrated luminosity of $9{\text{fb}}^{-1}$ collected in proton-proton collisions between 2011 and 2018 by the LHCb experiment, evidence is found for the decay $B_s^0\to \phi {\eta }^{\mathrm{\prime }}$ with $3.5\sigma$ significance. The branching ratio relative to the $B_s^0\to \phi \phi$ decay is determined to be $R=(3.56\pm 0.79\pm 0.18\pm 0.06)\times {10}^{-2}$. This corresponds to a branching fraction, $B(B_s^0\to \phi {\eta }^{\mathrm{\prime }})=(0.66\pm 0.15\pm 0.03\pm 0.02)\times {10}^{-6}$ where, in both cases, the first uncertainty is statistical, the second systematic, and the third due to external branching fractions.