A test of light-lepton universality in the rates of inclusive semileptonic $B$-meson decays at Belle II

Abstract

We present the first measurement of the ratio of branching fractions of inclusive semileptonic $B$-meson decays, $R(X_{e/\mu })=\mathcal{B}(B\to Xe\nu )/\mathcal{B}(B\to X\mu \nu )$, a precision test of electron-muon universality, using data corresponding to $189{\mathrm{fb}}^{-1}$ from electron-positron collisions collected with the Belle II detector. In events where the partner $B$ meson is fully reconstructed, we use fits to the lepton momentum spectra above $1.3\mathrm{GeV}/c$ to obtain $R(X_{e/\mu })=1.007\pm 0.009(\mathrm{stat})\pm 0.019(\mathrm{syst})$, which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.

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Cited by in corpus (6)

• When Energy Goes Missing: New Physics in $b\to s\nu \nu$ with Sterile Neutrinos
• Determination of $|V_{cb}|$ using ${\overline{B}}^0\to D^{\ast +}{\ell }^{-}{\bar{\nu }}_{\ell }$ decays with Belle II
• First Measurement of $R(X_{\tau /\ell })$ as an Inclusive Test of the $b\to c\tau \nu$ Anomaly
• Tests of light-lepton universality in angular asymmetries of $B^0\to D^{\ast -}\ell \nu$ decays
• Measurement of the Ratio of Partial Branching Fractions of Inclusive $\overline{B}\to X_u\ell \overline{\nu }$ to $\overline{B}\to X_c\ell \overline{\nu }$ and the Ratio of their Spectra with Hadronic Tagging
• Lepton universality tests and searches for new physics in charged current decays at Belle II