Test of light-lepton universality in τ decays with the Belle II experiment
Açıklama
We present a measurement of the ratio R-mu = B(tau(-) -> mu(-) (nu) over bar (mu)nu(tau))/B(tau(-) -> e(-)(nu) over bar (e)nu(tau)) of branching fractions 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 +/- 2 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 +/- 0.0007 +/- 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 +/- 0.0019, in agreement with the standard model expectation of unity.
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