$B$ Decays and Lepton Flavour (Universality) Violation

Abstract

LHCb found hints for physics beyond the standard model in $B\to K^{\ast }{\mu }^{+}{\mu }^{-}$, $B\to K^{\ast }{\mu }^{+}{\mu }^{-}/B\to K^{\ast }{e}^{+}{e}^{-}$ and $B_s\to \varphi {\mu }^{+}{\mu }^{-}$. In addition, the BABAR results for $B\to D^{(\ast )}\tau \nu$ and the CMS excess in $h\to {\tau }^{\pm }{\mu }^{\mp }$ also point towards lepton flavour (universality) violating new physics. While $B\to D^{(\ast )}\tau \nu$ and $h\to {\tau }^{\pm }{\mu }^{\mp }$ can be naturally explained by an extended Higgs sector, the probably most promising explanation for the $b\to s\mu \mu$ anomalies is a $Z^{\prime }$ boson. Furthermore, combining a 2HDM with a gauged $L_{\mu }-L_{\tau }$ symmetry allows for explaining the $b\to s{\mu }^{+}{\mu }^{-}$ anomalies and $h\to {\tau }^{\pm }{\mu }^{\mp }$ simultaneously, with interesting correlations to $\tau \to 3\mu$. In the light of these deviations from the SM we also discuss the possibilities of observing lepton flavour violating $B$ decays (e.g. $B\to K^{(\ast )}{\tau }^{\pm }{\mu }^{\mp }$ and $B_s\to {\tau }^{\pm }{\mu }^{\mp }$) in $Z^{\prime }$ models.

Comments: 8 pages, 3 figures, proceedings for "Les Rencontres de Physique de la Vallée d'Aoste, 2015"