Probing Lepton Universality with (Semi)-Leptonic B decays

The most recent measurements of the observables $R_{D^{(\ast )}}$ are in tension with the Standard Model offering hints of New Physics in $b\rightarrow c \ell \bar{\nu}{\ell}$ transitions. Motivated by these results, in this work we present an analysis on their $b\to u\mathrm{\ell }{\bar{\nu }}_{\mathrm{\ell }}$ counterparts (for $\mathrm{\ell }=e,\mu ,\tau$). 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 ($B^{-}\to {\mathrm{\ell }}^{-}{\bar{\nu }}_{\mathrm{\ell }}$, $\bar{B}\to \pi \mathrm{\ell }{\bar{\nu }}_{\mathrm{\ell }}$ and $\bar{B}\rightarrow \rho \ell \bar{\nu}{\ell}$)\mathrm{.}Hereakeyroleisplayedbytheleptonic$B$ 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 $\mathrm{\mid }{V}_{ub}\mathrm{\mid }$ while accounting simultaneously for New Physics contributions. Finally, we provide predictions for the branching fractions of yet unmeasured leptonic and semileptonic $B$ decays.