Tau and low multiplicity physics at Belle and Belle II

Abstract

The Belle and Belle II experiments have collected a 1.4 ab$^{-1}$ sample of $e^+e^-$ collision data at centre-of-mass energies near the $\Upsilon (nS)$ resonances, of which 424 fb$^{-1}$ were collected at Belle II in Run1 (2019--2022). We present a measurement of the lepton-flavour universality between electrons and muons, the search for the lepton-flavour violation decay $\tau^{-} \to \mu^{-}\mu^{+}\mu^{-}$, and a measurement of the $e^+e^- \to \pi^+\pi^-\pi^0$ cross section in the energy range from 0.62--3.5 GeV using an initial-state radiation technique, all performed with Belle II data.