This paper estimates how well the proposed electron–positron collider FCC‑ee could find axion‑like particles (ALPs) by looking for events with three photons. ALPs are hypothetical light particles that can couple to ordinary particles such as photons and Z bosons. The authors focus on a simple benchmark where the ALP interacts mainly with photons and study the process e+e− → aγ followed by a → γγ, which produces a clean three‑photon final state.

The team simulated collisions at all planned FCC‑ee energies: the Z pole (center‑of‑mass energy √s ≈ 91 GeV), the WW threshold (160 GeV), the ZH production peak (240 GeV), and the tt (top quark pair) threshold (340–365 GeV). They scanned ALP masses from about 5 GeV up to 320 GeV and used a standard Monte Carlo chain (MadGraph5_aMC@NLO, PYTHIA8) with a fast detector simulation (DELPHES). In the chosen benchmark the ALP coupling parameters were set so the particle decays mostly to two photons, and one million signal events were generated per mass point to estimate sensitivity.

At a basic level the signal is identified by finding two photons whose combined mass forms a narrow bump (the ALP) plus a third photon from the associated production. Production can proceed by exchanging either a photon or a Z boson. The Z resonance strongly enhances the production rate when the collider runs at the Z pole; the paper notes this enhancement can be roughly a factor of 1,300 compared with off‑resonance energies. There is a strong kinematic suppression as the ALP mass approaches the collision energy: the production rate scales like (1 − m_a^2/s)^3, so heavier ALPs are harder to produce at a given energy.

The main background is the ordinary Standard Model process that produces three photons directly. The authors treat that as the dominant, irreducible background and report that other backgrounds—such as misidentified particles—can be reduced by selection cuts. In the mass range and coupling scenario they study, the ALP decays promptly (very close to the collision point), so the analysis searches for prompt three‑photon events.