Abstract
Axion-like particles (ALPs) are pseudo Nambu–Goldstone bosons of spontaneously broken global symmetries in high-energy extensions of the Standard Model (SM). This makes them a prime target for future experiments aiming to discover new physics which addresses some of the open questions of the SM. While future high-precision experiments can discover ALPs with masses well below the GeV scale, heavier ALPs can be searched for at future high-energy lepton and hadron colliders. We discuss the reach of the different proposed colliders, focusing on resonant ALP production, ALP production in the decay of heavy SM resonances, and associate ALP production with photons, Z bosons or Higgs bosons. We consider the leading effective operators mediating interactions between the ALP and SM particles and discuss search strategies for ALPs decaying promptly as well as ALPs with delayed decays. Projections for the high-luminosity run of the LHC and its high-energy upgrade, CLIC, the future e^+e^- ring-colliders CEPC and FCC-ee, the future pp colliders SPPC and FCC-hh, and for the MATHUSLA surface array are presented. We further discuss the constraining power of future measurements of electroweak precision parameters on the relevant ALP couplings.
Highlights
Depending on the region in parameter space spanned by the Axion-like particles (ALPs) mass and couplings, the search strategies vary greatly
Searches for ALPs decaying into photons are motivated by the relation between the ALP coupling to gluons CGefGf and to photons Cγefγf in models addressing the strong CP problem, and from a practical point of view by the difficulty of observing light ALPs decaying into jets at hadron colliders
Di-jet searches at the LHC can provide bounds on heavy ALPs with masses ma > 1 TeV, whereas recent searches for a new vector resonance decaying into di-jets accompanied by hard initial state radiation pp → j Z → 3 j can be recast into limits on ALPs with masses below the TeV scale in the process pp → ja → 3 j [46,91,92]
Summary
Depending on the region in parameter space spanned by the ALP mass and couplings, the search strategies vary greatly. Experiments probing long-lived ALPs include helioscopes such as CAST [6], SUMICO [7,8], as well as observations from the evolution of red giant stars [9,10,11] and the Supernova SN1987a [12,13]. A set of cosmological constraints from the modification to big-bang nucleosynthesis, distortions of the cosmic microwave background and extragalactic background light measurements exclude a large region of this parameter space and are sensitive to very small ALP-photon couplings [14,15]. For intermediate ALP masses up to the GeV scale, collider experiments such as BaBar, CLEO, LEP and the LHC searching for missingenergy signals probe long-lived ALPs with non-negligible couplings to SM particles [16,17]. ALP couplings to charged leptons are constrained by searches for ALPs produced in the sun [22], the evolution of red giants
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