Tests of light-lepton universality in angular asymmetries of $B^0\to D^{\ast -}\ell \hat{I}½$ decays

Abstract

We present the first comprehensive tests of light-lepton universality in the angular distributions of semileptonic $B^0$-meson decays to charged spin-1 charmed mesons. We measure five angular-asymmetry observables as functions of the decay recoil that are sensitive to lepton-universality-violating contributions. We use events where one neutral $B$ is fully reconstructed in $\hat{I}\yen \left(4S\right)\to B\overline{B}$ decays in data corresponding to $189{\mathrm{fb}}^{-1}$ integrated luminosity from electron-positron collisions collected with the Belle II detector. We find no significant deviation from the standard model expectations.