Collider Probes of Axion-Like Particles

TL;DR

This work develops a comprehensive EFT framework for axion-like particles (ALPs) light enough to affect collider phenomenology, detailing their dimension-5 couplings to SM gauge bosons and fermions and the higher-dimensional Higgs portal operators that enable exotic decays h→Za and h→aa. It delivers complete one-loop calculations of ALP decays to photons, leptons, and hadrons, including chiral-based treatments for sub-GeV masses and the first calculation of a→πππ decays, and it studies how these decays inform collider signatures. The authors show that on-shell Higgs and Z decays at the LHC can probe ALP couplings deeply, even in regions where ALPs are long-lived or decay invisibly, and they map reach in the ALP parameter space across various final states, including connections to the muon g-2 anomaly. They also analyze electroweak precision constraints and provide future prospects for FCC-ee, illustrating a broad, complementary program to constrain or discover ALPs via exotic decays and precision measurements.

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 $a\toπππ$ 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 $(0.01-1) \mbox{TeV}^{-1}$, 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 $h\to Za$, $h\to aa$ and $Z\toγa$ 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 $a\toγγ$ and $a\to\ell^+\ell^-$ branching ratios are significantly less than 1. We also discuss constraints on the ALP parameter space from electroweak precision tests.

Figures (28)

• Figure 1: Representative one-loop Feynman diagrams contributing to the decay $a\to\gamma\gamma$. The internal boson lines represent charged $W$ bosons and the associated charged Goldstone fields. The last diagram contains the (gauge-dependent) self-energy $\Pi_{\gamma Z}(0)$. One also needs to include the on-shell wave-function renormalization factors for the external photon fields.
• Figure 2: Representative one-loop Feynman diagrams contributing to the decay $a\to\ell^+\ell^-$.
• Figure 3: ALP decay rates into pairs of SM particles obtained by setting the relevant effective Wilson coefficients to 1 (top), or by setting the ALP--fermion couplings to 1 and the ALP--boson couplings to $1/(4\pi^2)$ (bottom). The gray area between 1 and 3 GeV shows the region in which various exclusive hadronic (and difficult to calculate) decay channels such as $a\to\rho\rho$ open up. In this interval the rate $\Gamma(a\to\hbox{hadrons})$ is expected to interpolate between the black and red lines. The rates for decays into heavy-flavor jets are shown separately.
• Figure 4: Existing constraints on the ALP--photon (left) and ALP--electron coupling (right) derived from a variety of particle physics, astro-particle physics and cosmological observations. Several of these bounds are model dependent. The BaBar constraint in the right-hand plot assumes $c_{\mu\mu}\approx c_{ee}$, see (\ref{['LFuniv']}); otherwise, this is a bound on $|c_{\mu\mu}^{\rm eff}|$. See the text for more details.
• Figure 5: One-loop diagrams contributing to the anomalous magnetic moment of the muon.