Search for B^{0}→K^{*0}tau^{+}tau^{-} Decays at the Belle II Experiment

We present a search for the rare flavor-changing neutral-current decay B^{0}→K^{*0}τ^{+}τ^{-} 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 ϒ(4S) resonance. One of the B mesons produced in the ϒ(4S)→B^{0}B[over ¯]^{0} process is fully reconstructed in a hadronic decay mode, while its companion B meson is required to decay into a K^{*0} and two τ leptons of opposite charge. The τ leptons are reconstructed in final states with a single electron, muon, charged pion or charged ρ meson, and additional neutrinos. We set an upper limit on the branching fraction of B(B^{0}→K^{*0}τ^{+}τ^{-})<1.8×10^{-3} at the 90% confidence level, which is the most stringent constraint reported to date.