Collider constraints on axion-like particles

TL;DR

This paper reviews collider-based searches for axion-like particles (ALPs), highlighting current constraints and future prospects, with the LHC providing significant bounds on ALP masses and couplings through various decay channels.

Contribution

It summarizes the latest collider constraints on ALPs, compares different search channels, and projects future improvements in sensitivity over the next decade.

Findings

Exclusive PbPb collisions set the best limits for ALP masses 5-100 GeV.

Other channels are most sensitive for masses 100 GeV to 2.6 TeV.

Constraints on axion-photon couplings reach down to 0.05 TeV$^{-1}$, with future improvements expected.

Abstract

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 $a\to\gamma\gamma$ resonances in di-, tri-, and four-photon final states. Inclusive and exclusive diphotons in proton-proton and lead-lead collisions, pp, PbPb $\to a \to \gamma\gamma (+X)$, as well as exotic Z and Higgs boson decays, pp $\to \mathrm{Z},\mathrm{H}\to a\gamma \to 3\gamma$ and pp $\to \mathrm{H}\to aa \to 4\gamma$, have been analyzed. Exclusive searches in PbPb collisions provide the best exclusion limits for ALP masses $m_a\approx 5-$100 GeV, whereas the other channels are the most competitive ones over $m_a\approx 100$ GeV$-$2.6 TeV. Integrated ALP production cross sections up to $\sim$100 nb are excluded at 95% confidence level, corresponding to constraints on axion-photon couplings down to $g_{a\gamma}\approx$ 0.05 TeV$^{-1}$, over broad mass ranges. Factors of 10$-$100 improvements in these limits are expected at the LHC approaching $g_{a\gamma}\approx 10^{-3}$ TeV$^{-1}$ over $m_a\approx 1$ GeV$-$5 TeV in the next decade.