Tau and low multiplicity physics at Belle and Belle II

TL;DR

This paper reports on measurements and searches related to tau lepton decays and low-energy hadronic cross sections using extensive data from the Belle and Belle II experiments, focusing on lepton-flavour universality, lepton-flavour violation, and meson production.

Contribution

It provides new measurements of lepton-flavour universality, searches for lepton-flavour violation, and cross section measurements in the 0.62--3.5 GeV range using Belle II data, enhancing understanding of tau physics and low-energy QCD.

Findings

Measured lepton-flavour universality between electrons and muons.

Searched for and set limits on $ au^{-} o \mu^{-}\mu^{+}\mu^{-}$ decay.

Measured $e^+e^- o \pi^+\pi^-\pi^0$ cross section in the 0.62--3.5 GeV range.

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.