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 ${\rm Y}(nS)$ 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.