Test of light-lepton universality in $\tau$ decays with the Belle II experiment

Abstract

We present a measurement of the ratio $R_{\mu }=\mathcal{B}({\tau }^{-}\to {\mu }^{-}{\bar{\nu }}_{\mu }{\nu }_{\tau })/\mathcal{B}({\tau }^{-}\to e^{-}{\bar{\nu }}_e{\nu }_{\tau })$ of branching fractions $\mathcal{B}$ of the $\tau$ lepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKB $e^{+}{e}^{-}$ collider. The sample has an integrated luminosity of $362\pm 2{\text{fb}}^{-1}$ at a centre-of-mass energy of $10.58\text{GeV}$. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result, $R_{\mu }=0.9675\pm 0.0007\pm 0.0036$, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to the $W$ boson in $\tau$ decays of $0.9974\pm 0.0019$, in agreement with the standard model expectation of unity.

arXiv comment22 pages, 7 figures

References in corpus (15)

• An Introduction to PYTHIA 8.2
• The Precision Monte Carlo Event Generator KK For Two-Fermion Final States In e+e- Collisions
• Coherent Exclusive Exponentiation For Precision Monte Carlo Calculations
• SuperKEKB Collider
• The Belle II Core Software
• Matching perturbative and Parton Shower corrections to Bhabha process at flavour factories
• Charged-Higgs phenomenology in the Aligned two-Higgs-doublet model
• Lepton flavor violating Z' explanation of the muon anomalous magnetic moment
• Large-angle Bhabha scattering and luminosity at flavour factories
• Photon pair production at flavour factories with per mille accuracy
• Resonance Chiral Lagrangian Currents and Experimental Data for ${\tau }^{-}\to {\pi }^{-}{\pi }^{-}{\pi }^{+}{\nu }_{\tau }$
• Resonance chiral Lagrangian currents and tau decay Monte Carlo
• An Improved Parton Shower algorithm in QED
• Searching for Lepton Flavour (Universality) Violation and Collider Signals from a Singly-Charged Scalar Singlet
• Testing Lepton Flavor Universality with Pion, Kaon, Tau, and Beta Decays

Cited by in corpus (2)

• Flavor Physics at the CEPC: a General Perspective
• Lepton Flavor Violation: From Muon Decays to Muon Colliders