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-→ BB̅ collisions at a center-of-mass energy corresponding to the Υ(4S) resonance in samples with a combined integrated luminosity exceeding 1 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→ D(*)τν and B→ D(*)ℓν decays using semileptonic or hadronic reconstruction of the accompanying B-meson tag. Belle II and Belle measurements of angular observables in B→ D*ℓν 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.
Related
- Lepton Flavour Universality in Semileptonic B̅(s) → D(s)(∗) Transitions
- Improving lepton flavour universality tests with KL decays
- Test of lepton flavor universality with B̅ → D(*) τ− ν̅τ and B̅ → D(*) ℓ− ν̅ℓ decays at Belle II
- Simultaneous measurements of N-subjettiness observables in jets from gluons and light-flavour quarks, and in decays of boosted W bosons and top quarks
- Probing the neutrino mass through semileptonic decays
BibTeX
@misc{warburton2026measurements,
title = {{Measurements of lepton-flavour universality in semileptonic $B$ decays at Belle and Belle II}},
author = {Andreas Warburton},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.16617},
eprint = {2610.16617},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.16617},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Measurements of lepton-flavour universality in semileptonic B decays at Belle and Belle II
AU - Andreas Warburton
PY - 2026
DA - 2026/10/12
PB - arXiv
DO - 10.48550/arxiv.2610.16617
UR - https://arxiv.org/abs/2610.16617
AB - 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^-→ BB̅ collisions at a center-of-mass energy corresponding to the Υ(4S) resonance in samples with a combined integrated luminosity exceeding 1 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→ D^(*)τν and B→ D^(*)ℓν decays using semileptonic or hadronic reconstruction of the accompanying B-meson tag. Belle II and Belle measurements of angular observables in B→ D^*ℓν 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.
ER -