Collider constraints on axion-like particles

The current status and future prospects of searches for axion-like particles (ALPs) at colliders, mostly focused on the CERN LHC, are summarized. Constraints on ALPs with masses above a few GeV that couple to photons, as well as to Z or Higgs bosons, have been set at the LHC through searches for new a→γγ resonances in di-, tri-, and four-photon final states. Inclusive and exclusive diphotons in proton-proton and lead-lead collisions, pp, PbPb → a → γγ(+X), as well as exotic Z and Higgs boson decays, pp → Z,H→ aγ→ 3γ and pp → H→ aa → 4γ, have been analyzed. Exclusive searches in PbPb collisions provide the best exclusion limits for ALP masses mₐ≈ 5-100 GeV, whereas the other channels are the most competitive ones over mₐ≈ 100 GeV$-$2.6 TeV. Integrated ALP production cross sections up to ~100 nb are excluded at 95% confidence level, corresponding to constraints on axion-photon couplings down to gaγ≈ 0.05 TeV⁻¹, over broad mass ranges. Factors of 10$-$100 improvements in these limits are expected at the LHC approaching gaγ≈ 10⁻³ TeV⁻¹ over mₐ≈ 1 GeV$-$5 TeV in the next decade.