Discovering Axion-Like Particles with Photon Fusion at the ILC

Experimental searches for Axion-Like Particles (ALPs) which couple to the electroweak bosons span over a wide range of ALP masses, from MeV searches at beam-dump experiments, to TeV searches at the LHC. Here we examine an interesting range of parameter space in which the ALP couples only to hypercharge. In the GeV to hundreds of GeV mass range, the contribution of an ALP to light by light scattering can be significant. By making simple kinematic cuts, we show that the ILC running at $\sqrt{s}=250\mathrm{G}\mathrm{e}\mathrm{V}$ or $\sqrt{s} = 500,\rm{GeV}$ can discover ALPs in this range of masses with significantly smaller couplings to the SM than previous experiments, down to $g_{aBB} = 10^{-3},\rm{TeV}^{-1}$.