Speaker
Description
The Belle and Belle II experiments have collected a $1.6~\mathrm{ab}^{-1}$ sample of $e^+e^-$ collision data at centre-of-mass energies near the $\Upsilon(nS)$ resonances. This sample contains approximately 1.5 billion $e^+e^-\to \tau^+\tau^{-}$ events, which we use to search for lepton-flavour violating decays. We present searches for $\tau\to\ell\gamma$, tau decay to three charged leptons, $\tau^-\to K_{\rm S}^0\ell^{-}$, $\tau^-\to \ell^-\alpha$, where $\alpha$ is an invisible scalar particle. Further, 75% of these data are collected at a center-of-mass energy corresponding to the $\Upsilon(4S)$ resonance, which decays almost exclusively to $B\bar{B}$ pairs. We present results of several searches for non-standard-model $B\to K^{(*)}\tau \ell$ decays. Finally, we present searches for lepton-flavour-violation in bottomonium decay.