Speaker
Description
The Belle and Belle II experiments have collected a combined sample of $1.6~\mathrm{ab}^{-1}$ 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 and an $Z^{\prime}$ that decays to invisible particles. In addition, we search for the dark photon decay $A^{\prime}\to \mu^+\mu^-$ using the full Belle II data set of $500~fb^{-1}$ 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. The combined Belle and Belle II data 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, $\tau^-\to K_{\rm S}^0\ell^{-}$, and $\tau^-\to \ell^-\alpha$, where $\alpha$ is an invisible scalar particle.
