Orateur
Description
The Belle and Belle II experiment have collected samples of $e^+e^-$ collision data at centre-of-mass energies near the $\Upsilon(nS)$ resonances. These data have constrained kinematics and low multiplicity, which allow searches for dark sector particles in the mass range from a few MeV to 10 GeV. Using a 365 fb$^{-1}$ sample collected by Belle II, we search for inelastic dark matter. 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. Furthermore, the combined Belle and Belle II samples contain approximately 1.5 billion $e^+e^-\to \tau^+\tau^{-}$ events, which we use to search for lepton-flavour violating decays and make precision measurements of $\tau$ properties. We review our latest $\tau$ measurements.
