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

We present the results of a search for the charged-lepton-flavor violating decays $B^0 \rightarrow K^{ *0}\tau^\pm \ell^{ \mp}$, where $\ell^{ \mp}$ 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 $B^0 \rightarrow K^{ *0}\tau^\pm \ell^{ \mp}$ decays and set upper limits on the branching fractions in the range of $(2.9-6.4)\times10^{ -5}$ at 90% confidence level.

Categories: hep-ex