Bounds on axion-like particles from fully merged photons at the LHC

I show that, in the mass range $0.1\text{GeV}<m_a<6\text{GeV}$, the strongest current bound on axion-like particles (ALPs) coupled to photons comes from very high energy $a\gamma$ production at the LHC. In this regime, the two photons from the ALP decay are collimated and reconstructed as a single photon.