Probing the Axion--Photon--Dark Photon Interaction with LHC Mono-Photons and Fermi-LAT Gamma-Ray Lines

We investigate the axion–photon–dark photon interaction at the LHC and in gamma-ray line searches. By recasting the CMS mono-photon data, we derive a $95\mathrm{\%}$ CL exclusion reaching $g_{a{\gamma }^{\mathrm{\prime }}\gamma }\simeq 4.8\times {10}^{-4}{\mathrm{G}\mathrm{e}\mathrm{V}}^{-1}$ for dark photon masses $m_{{\gamma }^{\mathrm{\prime }}}\lesssim 100$ GeV, with sensitivity extending to $m_{{\gamma }^{\mathrm{\prime }}}\simeq 2.3$ TeV, where the effective theory approaches its validity limit. This improves upon the existing LEP mono-photon constraint by a factor of $3$ or more, depending on the dark photon mass. The projected HL-LHC sensitivity improves by a further factor of $\sim 2.7$ and extends the reach to $m_{{\gamma }^{\mathrm{\prime }}}\simeq 3$ TeV. The same interaction also induces $s$-wave annihilation of axions into $\gamma \gamma$. If the axion constitutes dark matter, Fermi-LAT gamma-ray line data become more constraining than the LHC for $m_a\gtrsim 160$ GeV at $m_{{\gamma }^{\mathrm{\prime }}}=300$ GeV, where the on-shell collider signal is kinematically suppressed. Combining collider and gamma-ray line constraints excludes a thermal relic axion across the axion mass range considered for this benchmark.