Lepton universality tests and searches for new physics in charged current decays at Belle II

Abstract

We present recent tests of lepton universality as crucial probes of the Standard Model in semileptonic $B$-meson decays at Belle II. All presented analyses use a data sample collected at the ${\rm Y}(4S)$ resonance by the Belle II experiment corresponding to an integrated luminosity of $189{\mathrm{fb}}^{-1}$. We report three analyses that probe light-lepton universality: The first measurement of a complete set of five angular asymmetries using ${\bar{B}}^0\to D^{\ast +}{\ell }^{-}{\bar{\nu }}_{\ell }$ decays, followed by a measurement of the forward-backward asymmetry in untagged ${\bar{B}}^0\to D^{\ast +}{\ell }^{-}{\bar{\nu }}_{\ell }$ decays, and finally, measurements of the branching-fraction ratios $R(D_{e/\mu }^{\ast +})=\mathcal{B}({\bar{B}}^0\to D^{\ast +}{e}^{-}{\bar{\nu }}_e)/\mathcal{B}({\bar{B}}^0\to D^{\ast +}{\mu }^{-}{\bar{\nu }}_{\mu })$, using the same untagged data set, along with the first measurement of the inclusive light-lepton ratio $R(X_{e/\mu })=\mathcal{B}(\bar{B}\to X{e}^{-}{\bar{\nu }}_e)/\mathcal{B}(\bar{B}\to X{\mu }^{-}{\bar{\nu }}_{\mu })$. Furthermore, we present two tests of heavy-to-light lepton universality. In the first test, we report the measurement of the branching-fraction ratio $R(D_{\tau /\ell }^{\ast })$ using hadronic tagging. In the second test, we present the first measurement of the inclusive ratio $R(X_{\tau /\ell })$. All presented results are consistent with their corresponding Standard Model predictions and, where applicable, with the experimental world averages.

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