Kavli Affiliate: T. Higuchi
| First 5 Authors: Belle II Collaboration, I. Adachi, K. Adamczyk, L. Aggarwal, H. Aihara
| Summary:
We present a measurement of the ratio $R_mu = mathcal{B}(tau^-to
mu^-barnu_munu_tau) / mathcal{B}(tau^-to e^-barnu_enu_tau)$ 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 fb$^{-1}$ at
a centre-of-mass energy of 10.58 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.
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