Probing the ALP--Photon--Dark Photon Interaction through the Mono-Photon plus Missing-Energy Signature at a Muon Collider

Abstract

The dark axion portal provides a well-motivated framework for new physics beyond the Standard Model (SM), in which an axion-like particle (ALP) couples simultaneously to a photon and a dark photon, leading to distinctive collider signatures. In this work, we investigate the sensitivity to the $a$--$γ$--$γ'$ interaction via the mono-photon channel arising from $μ^{+}μ^{-} \to a γ'$ with $a \to γγ'$ at the $1.5~\mathrm{TeV}$ muon collider with an integrated luminosity of $\mathcal{L}=500~\mathrm{fb}^{-1}$ for the polarized and unpolarized beam configurations, $P(μ^+,μ^-)=(-100\%,+100\%)$ and $P(μ^+,μ^-)=(0,0)$, using detector-level simulation. The signal is characterized by a single energetic photon accompanied by missing energy as the dark photons escape detection. The projected sensitivity reaches the $\mathcal{O}(10^{-5})~\mathrm{GeV}^{-1}$ level for ALP masses in the range $1~\mathrm{GeV}\leq m_a\leq1200~\mathrm{GeV}$ for $P(μ^+,μ^-)=(-100\%,+100\%)$, while for $P(μ^+,μ^-)=(0,0)$ the projected sensitivity reaches the $\mathcal{O}(10^{-4})~\mathrm{GeV}^{-1}$ level for the same ALP mass range. These results demonstrate that the mono-photon signature at a muon collider provides a sensitive probe of the $a$--$γ$--$γ'$ interaction over a wide mass range.