Searching for Lepton Flavour Violation at Belle II

par Clotilde Lemettais (Laboratoire de Physique de Clermont Auvergne)

vendredi 13 mars 2026, 14:30 → 15:30 Europe/Paris

Amphi Recherche

Lepton Flavour Violation (LFV) being forbidden in the Standard Model (SM), LFV searches represent a powerful probe for New Physics. In addition, many SM extensions predict lepton flavour violation with enhanced branching fractions for decays involving the third generation of leptons, reaching rates close to the current experimental sensitivities.

The Belle II experiment, located at the SuperKEKB $e^{+}e^{-}$ collider at KEK, collected a total of $365\,\mathrm{fb}^{-1}$ of data at the $\Upsilon(4S)$ resonance ($10.58\,\mathrm{GeV}$) during its first data taking period, from 2019 to 2022. The Run~1 dataset corresponding to around 390 millions of $B\bar{B}$ pairs and 380 millions of $\tau$ lepton pairs produced in a clean environment and almost hermetic detector, provides an ideal environment to search for a wide variety of lepton-flavour violating decays in both $B$ meson and $\tau$ lepton decays. Indeed the $B$-tagging techniques developed at $B$ factories allow excellent constraints on missing energy in $B$ decays by making use of the particular geometry and initial state knowledge; and the almost background free environment reached in $\tau$ decays is perfectly suited for forbidden processes studies.

This presentation covers a general overview of LFV searches at Belle~II in $B$ and $\tau$ decays. The first search for $B^{0}\to K_{S}^{0}\,\tau\ell$, where $\ell$ is an electron or a muon, is presented and the search for $B^{0}\to K^{*0}\,\tau\ell$ is detailed. Searches for $\tau\to \ell K_{S}^{0}$, $\tau\to \mu\mu\mu$ and $\tau\to e\ell\ell$ decays, resulting in new world-leading limits, are also presented.