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

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 a top quark decaying into a light quark with a Higgs or gauge boson in the final state, i.e., t→ q(h,V) with V=γ, Z, g, h→ ', and → ' γ. To achieve the assumption that the induced FCNCs are all from quantum loops, we consider the scotogenic mechanism, where a Z₂ symmetry is introduced and only new particles carry an odd Z₂ 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→ c (h, Z), h→ μτ, and τ→ γ decays can be enhanced up to the expected sensitivities in experiments. The branching ratios of h→ μ⁺ μ⁻/τ⁺ τ⁻ from only new physics effects can reach up to O(10⁻³). Intriguingly, the resulting muon $g-2$ can fit the combined data within $2$ standard deviations, whereas the electron $g-2$ can have either sign with a magnitude of 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.