Measurements of lepton-flavour universality in semileptonic $B$ decays at Belle and Belle II

Abstract

Tests of lepton-flavour universality in semileptonic \(B\)-meson decays provide sensitive probes of physics beyond the Standard Model. The Belle and Belle II experiments, respectively operating on the KEKB and SuperKEKB colliders in Japan, have collected \(e^+e^-\to B\bar{B}\) collisions at a center-of-mass energy corresponding to the \(Υ(4S)\) resonance in samples with a combined integrated luminosity exceeding \(1\,\mathrm{ab}^{-1}\). These data, with low particle multiplicity, constrained initial-state kinematics, and excellent lepton identification, provide an ideal environment to study lepton-flavour universality in semileptonic decays of \(B\) mesons. New results on the ratios of semitauonic \(B\)-decay rates compared to analogous light-lepton rates are presented. These include new Belle II measurements of branching-fraction ratios \(R(D^{(*)})\) involving exclusive \(B\to D^{(*)}τν\) and \(B\to D^{(*)}\ellν\) decays using semileptonic or hadronic reconstruction of the accompanying \(B\)-meson tag. Belle II and Belle measurements of angular observables in \(B\to D^*\ellν\) modes that test light-lepton universality and the Lorentz structure of the decay are also presented. The measurements are statistically limited and individually consistent with Standard Model expectations. The new \(R(D^{(*)})\) results are also compatible with experimental world averages, which continue to exhibit tension with Standard Model predictions. Belle II has now accumulated an \(Υ(4S)\) data sample exceeding that of Belle, offering substantial opportunities for new analysis approaches and heightened precision.