Speaker
Description
An abelian extension of the Standard Model (SM) based on the introduction of a new abelian gauge group $U(1)^\prime$ is considered, with the corresponding neutral gauge boson $Z^\prime$ having flavour violating couplings to quarks and leptons. The fermion content is the same as in the SM, except for the addition of three right-handed neutrinos. The model, proposed by Aebischer, Buras, Cerdà-Sevilla and De Fazio, describes the couplings of $Z^\prime$ to fermions in terms of three rational parameters $\epsilon_{1,2,3}$ that sum to zero when imposing the cancellation of gauge anomalies. Each $\epsilon_i$ is common to all fermions of a given generation, a feature producing correlations among quark and lepton observables.
Within this framework, I investigated $b \to s \ell_1^- \ell_2^+$ transitions, both in the lepton flavour conserving case ($\ell_1=\ell_2$) and in the lepton flavour violating case ($\ell_1 \neq \ell_2$). In the former, small deviations from SM predictions were found, reflecting a feature of the model in which quark and lepton sectors prevent each other to manifest large discrepancies with respect to SM.
Additionally, I studied the correlations between rare $B$ and $B_s$ decays and the leptonic processes $\tau^- \to \mu^- \mu^+ \mu^-$, $\mu^- \to e^- \gamma$, $\mu^- \to e^- e^+ e^-$ and $ \mu^- \to e^-$ conversion in nuclei. The conducted analysis shows that the current experimental upper bounds on these channels play an increasingly important role in constraining the branching ratios of lepton flavor violating $B$ and $B_s$ decays. In particular, the bound on $\tau^- \to \mu^- \mu^+ \mu^-$ does not impose any significant restriction, whereas the other three channels set progressively more stringent limits.
