Z mediated flavor changing neutral currents with a fourth vectorlike family

Abstract

We discuss $Z$ mediated flavor changing neutral currents within a model where the hierarchical quark and lepton masses are explained via a fourth vectorlike family, together with a scalar sector consisting of two Higgs doublets augmented by a gauge singlet scalar field that spontaneously breaks an extra global $U(1{)}^{\ensuremath{'}}$ symmetry. The $Z$ mediated flavor violating interactions arise from the mixings between the SM fermions and the vectorlike fermions, where the mixing is discussed in an analytic approximation and also exactly numerically. We first discuss charged lepton flavor violating (CLFV) $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$, $\ensuremath{\tau}\ensuremath{\rightarrow}3\ensuremath{\mu}$ and $Z\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau}$ decays and find that they cannot significantly constrain the masses of charged vectorlike leptons. However, the 790 GeV mass bound arising from collider searches on vectorlike lepton doublets can set further constraints on the model parameter space. We also consider rare $t\ensuremath{\rightarrow}cZ$ decays as well as unitarity violation in the CKM mixing in order to constrain the quark sector of the model under consideration.