$B$ Decays and Lepton Flavour (Universality) Violation

TL;DR

This paper reviews recent experimental hints of lepton flavor universality violation in B decays and discusses theoretical models, including extended Higgs sectors and Z' bosons, that could explain these anomalies and predict new observable phenomena.

Contribution

It synthesizes current experimental anomalies and proposes unified theoretical frameworks involving Z' bosons and extended Higgs sectors to explain lepton flavor violation in B decays.

Findings

Hints of lepton flavor universality violation in B decays.

Extended Higgs sectors can explain some anomalies.

Z' bosons provide a promising explanation for b→sμμ anomalies.

Abstract

LHCb found hints for physics beyond the standard model in $B\to K^*\mu^+\mu^-$, $B\to K^*\mu^+\mu^-/B\to K^*e^+e^-$ and $B_s\to\phi\mu^+\mu^-$. In addition, the BABAR results for $B\to D^{(*)}\tau\nu$ and the CMS excess in $h\to\tau^\pm\mu^\mp$ also point towards lepton flavour (universality) violating new physics. While $B\to D^{(*)}\tau\nu$ 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 $Z'$ 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 $\tau\to3\mu$. In the light of these deviations from the SM we also discuss the possibilities of observing lepton flavour violating $B$ decays (e.g. $B\to K^{(*)}\tau^\pm\mu^\mp$ and $B_s\to\tau^\pm\mu^\mp$) in $Z^\prime$ models.