Search for $B^0\to K^{\ast 0}{\tau }^{+}{\tau }^{-}$ decays at the Belle II experiment

We present a search for the rare flavor-changing neutral-current decay $B^0\to K^{\ast 0}{\tau }^{+}{\tau }^{-}$ with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb${}^{-1}$ data sample recorded at the center-of-mass energy of the $\mathrm{{\rm Y}}(4S)$ resonance. One of the $B$ mesons produced in the $\mathrm{{\rm Y}}(4S)\to B^0{\bar{B}}^0$ process is fully reconstructed in a hadronic decay mode, while its companion $B$ meson is required to decay into a $K^{\ast 0}$ and two $\tau$ leptons of opposite charge. The $\tau$ leptons are reconstructed in final states with a single electron, muon, charged pion or charged $\rho$ meson, and additional neutrinos. We set an upper limit on the branching ratio of $BR(B^0\to K^{\ast 0}{\tau }^{+}{\tau }^{-})<1.8\times {10}^{-3}$ at the 90% confidence level, which is the most stringent constraint reported to date.