Sensitivity of the FCC-ee to decay of an axion-like particle into two photons

Abstract: The decay of the $Z$ boson into axion-like particles (ALP) at the $Z$-pole run of the proposed CERN FCC-ee collider is investigated. We perform a detailed study of final states with three photons or with a single photon and missing energy, yielding an evaluation of the accessible values of the ALP coupling to the photon for ALP masses between 0.1 and 85 GeV. Special attention is paid to the experimental implications of detecting the three-photon signature for the ALP mass region below 10~GeV. The FCC-ee run will be able to detect the ALP for couplings down to a few $10^{-3}~\mathrm{TeV}^{-1}$ over the considered mass range in an effective model where the ALP only couples to the U(1) boson of the Standard Model.