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 $a$--$\gamma$--${\gamma }^{\prime }$ interaction via the mono-photon channel arising from ${\mu }^{+}{\mu }^{-}\to a{\gamma }^{\prime }$ with $a\to \gamma {\gamma }^{\prime }$ 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, $P({\mu }^{+},{\mu }^{-})=(-100\%,+100\%)$ and $P({\mu }^{+},{\mu }^{-})=(0,0)$, 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 $1\mathrm{GeV}\le m_a\le 1200\mathrm{GeV}$ for $P({\mu }^{+},{\mu }^{-})=(-100\%,+100\%)$, while for $P({\mu }^{+},{\mu }^{-})=(0,0)$ 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 $a$--$\gamma$--${\gamma }^{\prime }$ interaction over a wide mass range.

18 pages, 7 figures, 3 tables