Probing Lepton Universality with (Semi)-Leptonic B decays

Abstract

The most recent measurements of the observables $R_{D^{(\ast )}}$ are in tension with the Standard Model offering hints of New Physics in $b\to c\ell {\bar{\nu }}_{\ell }$ transitions. Motivated by these results, in this work we present an analysis on their $b\to u\ell {\bar{\nu }}_{\ell }$ counterparts (for $\ell =e,\mu ,\tau$). Our study has three main objectives. Firstly, using ratios of branching fractions, we assess the effects of beyond the Standard Model scalar and pseudoscalar particles in leptonic and semileptonic $B$ decays ($B^{-}\to {\ell }^{-}{\bar{\nu }}_{\ell }$, $\bar{B}\to \pi \ell {\bar{\nu }}_{\ell }$ and $\bar{B}\to \rho \ell {\bar{\nu }}_{\ell }$). Here a key role is played by the leptonic $B$ processes, which are highly sensitive to new pseudoscalar interactions. In particular, we take advantage of the most recent measurement of the branching fraction of the channel $B^{-}\to {\mu }^{-}{\bar{\nu }}_{\mu }$ by the Belle collaboration. Secondly, we extract the CKM matrix element $|V_{ub}|$ while accounting simultaneously for New Physics contributions. Finally, we provide predictions for the branching fractions of yet unmeasured leptonic and semileptonic $B$ decays.

12 pages, 6 figures. To appear in Proceedings for the 15th International Workshop on Tau Lepton Physics