General Formulas for Loop-Induced Decays of $A \to Zγγ$ and Their Applications

Abstract: Within the framework of the Standard Model Higgs extensions, including the Two-Higgs-Doublet Model with vector-like fermions and the Triplet-Higgs Model, we derive general one-loop contributions to the rare decay process $A \rightarrow Z \gamma \gamma$. The analytical expressions are formulated with Passarino-Veltman scalar functions, which represent the scalar coefficients of one-loop Lorentz-covariant tensor integrals. These functions are written in accordance with the input conventions of the {\tt LoopTools} and {\tt Collier} packages, facilitating the efficient numerical generation of decay rates and their distributions using these computational tools. As part of our phenomenological study, we examine the branching ratios of this decay channel within the viable parameter space of the considered models. Our results indicate that the branching ratio can reach $\mathcal{O}(10^{-4})$ in the Two-Higgs-Doublet Model and $\mathcal{O}(10^{-2})$ in the Triplet-Higgs Model at specific points within the allowed parameter regions. Furthermore, the inclusion of vector-like fermions in the loop leads to a significant modification of the partial decay rates, with an observed variation of approximately $10\%$. Additionally, we explore the dependence of the branching ratios on key model parameters, including the charged Higgs mass, mixing angles, and the soft-breaking scale, providing deeper insights into the phenomenological implications of these Higgs-extended frameworks.