Light Axion-Like Particles at Future Lepton Colliders

TL;DR

The paper investigates light ALPs coupled to electroweak gauge bosons within an EFT framework characterized by two Wilson coefficients $C_W$, $C_B$ and decay constant $f_a$. It shows that long-lived ALPs yield strong mono-$V$ signatures (notably mono-photon at a Tera-$Z$ facility) while heavier or promptly decaying ALPs are probed via non-resonant vector-boson scattering (light-by-light and EW VBS). The authors perform detailed collider studies for future $e^+e^-$ (CEPC/FCC-ee) and multi-TeV $\mu^+\mu^-$ colliders, using MG5_aMC@NLO and Delphes, and provide combined 95% CL constraints on ALP couplings to EW bosons, highlighting the complementarity of mono-$V$ and VBS channels. They compare projected bounds against LEP and LHC results, finding substantial improvements, especially in the ALP long-lived regime, with Tera-$Z$ and high-luminosity runs delivering the strongest sensitivity. Overall, mono-$V$ and non-resonant VBS offer robust, mass-independent probes of ALP–gauge-boson interactions at future lepton colliders, complementing resonant searches at higher masses.

Abstract

Axion-like particles (ALPs) are well-motivated extensions of the Standard Model (SM) that appear in many new physics scenarios, with masses spanning a broad range. In this work, we systematically study the production and detection prospects of light ALPs at future lepton colliders, including electron-positron and multi-TeV muon colliders. At lepton colliders, light ALPs can be produced in association with a photon or a $Z$ boson. For very light ALPs ($m_a < 1$ MeV), the ALPs are typically long-lived and escape detection, leading to a mono-$V$ ($V = γ, Z$) signature. In the long-lived limit, we find that the mono-photon channel at the Tera-$Z$ stage of future electron-positron colliders provides the strongest constraints on ALP couplings to SM gauge bosons, $g_{aVV}$, thanks to the high luminosity, low background, and resonant enhancement from on-shell $Z$ bosons. At higher energies, the mono-photon cross section becomes nearly energy-independent, and the sensitivity is governed by luminosity and background. At multi-TeV muon colliders, the mono-$Z$ channel can yield complementary constraints. For heavier ALPs ($m_a > 100$ MeV) that decay promptly, mono-$V$ signatures are no longer valid. In this case, ALPs can be probed via non-resonant vector boson scattering (VBS) processes, where the ALP is exchanged off-shell, leading to kinematic deviations from SM expectations. We analyze constraints from both light-by-light scattering and electroweak VBS, the latter only accessible at TeV-scale colliders. While generally weaker, these constraints are robust and model-independent. Our combined analysis shows that mono-$V$ and non-resonant VBS channels provide powerful and complementary probes of ALP-gauge boson interactions.

Figures (20)

• Figure 1: The decay length contours of the ALP in the $(m_a,\, g_{a\gamma\gamma})$ plane. Above the purple contours, the ALP can be identified through its decay into photon pairs, while below the contours, the ALP remains invisible. For reference, we also display the constraints on $g_{a\gamma\gamma}$ from $\Upsilon \to \gamma +{\rm inv.}$CrystalBall:1990xecMasso:1995tw using Crystal Ball data and a recast of the mono-photon search at the LEP 189 GeV OPAL:2000puu. A hard cut at $L_D=3.6\,$ m is applied to ensure the validity of the results.
• Figure 2: Representatives Feynman diagrams for the signal (a) and backgrounds (b--d) for the mono-photon production at lepton colliders.
• Figure 3: The cross-sections of the signal and background processes for (a) mono-photon and (b) mono-$Z$ production as functions of the collider energy. The signal cross-sections are computed with $C_W/f_a=C_B/f_a=1\,{\rm TeV}^{-1}$. The mono-photon production is evaluated with universal photon cuts $p_{T,\gamma} > 10$ GeV and $|\eta_\gamma| < 2.5$. The final-state leptons are required to be outside the detector coverage ($|\eta_\ell| > 2.5$) for the $\gamma/Z$ exchange process $\ell^+ \ell^- \to V + \ell^+ \ell^-$. Additionally, an invariant mass cut of $M_{\ell\ell,\nu\nu} > 150$ GeV is applied to suppress on-shell $Z \to \ell^+ \ell^- / \nu_\ell \bar{\nu}_\ell$ decays in the $\gamma/Z$ and $W$ exchange channels.
• Figure 4: Current constraints on $C_W$ and $C_B$ in units of $f_a / {\rm TeV}$. The red contour is a recast of the mono-photon search at the LEP 189 GeV OPAL:2000puu, the orange and green bounds are derived from the $\Upsilon$CrystalBall:1990xecMasso:1995tw and $Z$Brivio:2017ije decays, respectively. The purple contour shows constraints from non-resonant VBS at CMS Run 2 CMS:2020fqzCMS:2020gfhCMS:2020ypoCMS:2021gme, while the dashed contour denotes the projected sensitivity at the 3 ${\rm ab}^{-1}$ HL-LHC Bonilla:2022pxu.
• Figure 5: (a) Cross-sections of the signal and background processes for mono-photon production at $e^+e^-$ colliders with $\sqrt{s} \approx M_Z$, applying a baseline cut of $p_{T,\gamma} > 15$ GeV and $|\eta_\gamma| < 2.5$. (b) Normalized $p_{T,\gamma}$ distribution of the photon produced by the signal and background processes at a 91.2 GeV $e^+e^-$ collider.