Search for lepton flavor-violating decay modes $B^0\to K^{\ast 0}{\tau }^{\pm }{\mathrm{\ell }}^{\mp }$ ($\mathrm{\ell }=e,\mu$) with hadronic B-tagging at Belle and Belle II

We present the results of a search for the charged-lepton-flavor violating decays $B^0\to K^{\ast 0}{\tau }^{\pm }{\mathrm{\ell }}^{\mp }$, where ${\mathrm{\ell }}^{\mp }$ is either an electron or a muon. The results are based on 365 fb${}^{-1}$ and 711 fb${}^{-1}$ datasets collected with the Belle II and Belle detectors, respectively. We use an exclusive hadronic $B$-tagging technique, and search for a signal decay in the system recoiling against a fully reconstructed $B$ meson. We find no evidence for $B^0\to K^{\ast 0}{\tau }^{\pm }{\mathrm{\ell }}^{\mp }$ decays and set upper limits on the branching fractions in the range of $(2.9-6.4)\times {10}^{-5}$ at 90% confidence level.