Abstract
Axion-like particles (ALPs), which are gauge-singlets under the Standard Model (SM), appear in many well-motivated extensions of the SM. Describing the interactions of ALPs with SM fields by means of an effective Lagrangian, we discuss ALP decays into SM particles at one-loop order, including for the first time a calculation of the a → πππ decay rates for ALP masses below a few GeV. We argue that, if the ALP couples to at least some SM particles with couplings of order (0.01 − 1) TeV−1, its mass must be above 1 MeV. Taking into account the possibility of a macroscopic ALP decay length, we show that large regions of so far unconstrained parameter space can be explored by searches for the exotic, on-shell Higgs and Z decays h → Za, h → aa and Z → γa in Run-2 of the LHC with an integrated luminosity of 300 fb−1. This includes the parameter space in which ALPs can explain the anomalous magnetic moment of the muon. Considering subsequent ALP decays into photons and charged leptons, we show that the LHC provides unprecedented sensitivity to the ALP-photon and ALP-lepton couplings in the mass region above a few MeV, even if the relevant ALP couplings are loop suppressed and the a → γγ and a → ℓ+ℓ− branching ratios are significantly less than 1. We also discuss constraints on the ALP parameter space from electroweak precision tests.
Highlights
New pseudoscalar particles with masses below the electroweak scale appear frequently in well-motivated extensions of the Standard Model (SM)
Taking into account the possibility of a macroscopic Axion-like particles (ALPs) decay length, we show that large regions of so far unconstrained parameter space can be explored by searches for the exotic, on-shell Higgs and Z decays h → Za, h → aa and Z → γa in Run-2 of the LHC with an integrated luminosity of 300 fb−1
This improvement results from the fact that we consider decays of on-shell Higgs or Z bosons in a parameter region where the ALP decays in SM particles before it leaves the detector
Summary
New pseudoscalar particles with masses below the electroweak scale appear frequently in well-motivated extensions of the Standard Model (SM). In this paper we present a comprehensive analysis of the on-shell Higgs decay modes h → Za and h → aa as well as the on-shell Z-boson decay Z → γa starting from a general effective Lagrangian for a gauge-singlet ALP interacting with SM fields We show that these decays can be used to probe the ALP couplings to SM particles in regions of parameters space inaccessible to any other searches. The extraordinary reach of on-shell h → Za, h → aa and Z → γa searches in constraining the ALP couplings to photons, charged leptons and heavy quarks allows us to improve existing bounds derived from searches for e+e− → γa at LEP and pp → γa at LHC [23,24,25] by up to six orders of magnitude This improvement results from the fact that we consider decays of on-shell Higgs or Z bosons in a parameter region where the ALP decays in SM particles before it leaves the detector. Technical details of our calculations are relegated to four appendices
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