Top-quark FCNC decays, LFVs, lepton $g-2$, and $W$ mass anomaly with inert charged Higgses

Abstract

The observed flavor-changing neutral-current (FCNC) processes in the standard model (SM) arise from the loop diagrams involving the weak charged currents mediated by the $W$-gauge boson. Nevertheless, the top-quark FCNCs and lepton-flavor violating processes resulting from the same mechanism are highly suppressed. We investigate possible new physics effects that can enhance the suppressed FCNC processes, such as $t\to q(h,V)$ with $V=\gamma,Z,g$, $h\to \ell \ell'$, and $\ell\to \ell' \gamma$. To achieve the assumption that the induced-FCNCs are all from quantum loops, we consider the scotogenic mechanism, where a $Z_2$ symmetry is introduced and only new particles carry an odd $Z_2$ parity. With the extension of the SM to include an inert Higgs doublet, an inert charged Higgs singlet, a vector-like singlet quark, and two neutral leptons, it is found that, with relevant constraints taken into account, the $t\to c (h, Z)$, $h\to \mu\tau$, and $\tau\to \ell \gamma$ decays can be enhanced up to the expected sensitivities in experiments. The branching ratios of $h\to \mu^+ \mu^-/\tau^+ \tau^-$ from only new physics effects can reach up to ${\cal O}(10^{-3})$. Intriguingly, the resulting muon $g-2$ can fit the combined data within $2\sigma$ errors, whereas the electron $g-2$ can have either sign with a magnitude of ${\cal O}(10^{-13}-10^{-12})$. In addition, we examine the oblique parameters in the model and find that the resulting $W$-mass anomaly observed by CDF II can be accommodated.