Phenomenology of Long-Lived Dark Photons and Axion-Like Particles in a Mixed Portal Framework

Abstract

We investigate the phenomenology of a dark photon \(A'\) and an axion-like particle (ALP) ,\(a\), connected through a mixed portal framework which simultaneously allows the conventional visible decay $(A'\rightarrow f\bar f)$ and the exotic cascade process $(A'\rightarrow a\gamma\rightarrow3\gamma)$. We derive the relevant decay widths, branching ratios, and Lorentz-boosted decay lengths, and introduce a dominance parameter \(D=\Gamma(A'\to a\gamma)/\Gamma_{\rm SM}\) to distinguish Standard Model-dominated and cascade-dominated regions, with the transition occurring at $(D=1)$. A detailed analysis of both light $(0.1\leq m_{A'}\leq10~{\rm GeV})$ and heavy $(10\leq m_{A'}\leq100~{\rm GeV})$ dark-photon scenarios shows that the exotic channel can substantially modify the expected dark-photon signatures, transforming otherwise long-lived or detector-stable states into experimentally accessible displaced multi-photon events. In addition, the ALP sector itself may exhibit long-lived particle behavior, leading to distinct displaced diphoton signatures. Our results show that mixed dark-photon-ALP portals offer a rich LLP phenomenology that can be explored at future high-luminosity lepton colliders such as the FCC-ee.