Speaker
Description
The Belle and Belle~II experiments have collected a $1.4~\mathrm{ab}^{-1}$ sample of $e^+e^-$ collision data at centre-of-mass energies near the $\Upsilon(nS)$ resonances. This sample contains approximately 1.3 billion $e^+e^-\to \tau^+\tau^{-}$ events, which we use to search for lepton-flavour violating decays. We present searches for $\tau-\to \mu^-\mu^-\mu^+$, $\tau^-\to\Lambda\pi^-$, and $\tau^-\to \bar{\Lambda}\pi^-$. We also present world leading measurements of the $\tau$ mass and lepton-flavour universality in $\tau\to\ell\nu\bar{\nu}$ decay, where $\ell$ is an electron or a muon. These data have constrained kinematics and low multiplicity, which also allow searches for dark sector particles in the mass range from a few MeV to 10 GeV. Using a 426 fb$^{-1}$ sample collected by Belle II, we search for inelastic dark matter accompanied by a dark Higgs. Using a 711 fb$^{-1}$ sample collected by Belle, we search for $B\to h + \mathrm{invisible}$ decays, where $h$ is a $\pi$, $K$, $D$, $D_{s}$ or $p$, and $B\to Ka$, where $a$ is an axion-like particle.
