Probing the CP Property of ALP-photon Interactions at Future Lepton Colliders

Abstract

We investigate a charge-parity (CP) odd axion-like particle (ALP) featuring simultaneous CP-conserving ($a F_{μν}\tilde{F}^{μν}$) and CP-violating ($a F_{μν}F^{μν}$) ALP-photon interactions at future lepton colliders. The ALP signal is studied in the process $e^+e^- \to e^+e^- a \to e^+e^- γγ$, where the CP structure of the interaction can be probed using the azimuthal angle difference between the final-state electrons, $Δφ_{ee}$. We show that the projected sensitivity to the ALP-photon couplings can reach $\mathcal{O}(10^{-3})~\mathrm{TeV}^{-1}$, exceeding current constraints from the electron electric dipole moment ($e$EDM). Because purely CP-conserving, purely CP-violating, and mixed interactions generate distinct $Δφ_{ee}$ distributions, a binned likelihood analysis of this observable enables an efficient discrimination of the ALP interaction structure. In particular, if the CP-conserving and CP-violating couplings are comparable--as motivated by possible symmetry considerations--the interference pattern in the $Δφ_{ee}$ distribution allows future lepton colliders to identify CP violation in the ALP sector once a signal is observed. For scenarios where the two couplings differ significantly, increasing the integrated luminosity substantially improves the sensitivity to CP-violating effects.

Figures (8)

• Figure 1: The Feynman diagrams for $e^+e^-\to e^+e^-a$ channel. The left panel represents the ALP production via vector boson fusion, and the right panel illustrates the ALP production in $s$--channel.
• Figure 2: Left panel: The cross-section for the process $\sigma (e^+ e^- \to e^+ e^- a)$ is depicted as a function of the ALP mass $m_a$ considering three distinct contributions: Vector Boson Fusion (VBF) represented in red, s-channel denoted in green, and the combined effect of all contributing diagrams shown as dot-dashed blue. The values for both $C_\gamma$ and $\tilde{C}_\gamma$ are set to be 0.1. Right panel: Azimuthal angular difference ($\Delta\phi_{ee}$) distributions for $e^+e^-$ pairs, assuming an ALP mass $m_a = 5$ GeV.
• Figure 3: The energy distribution and decay length for the ALPs at future lepton colliders with $\sqrt{s}=240$ GeV.
• Figure 4: Representative Feynman diagrams for the SM background processes contributing to $e^+ e^- \to e^+ \, e^- \,\gamma \,\gamma$. Additional diagrams, including those with photon exchange in the final state due to identical diphotons, are topologically equivalent to the illustrated cases and are omitted for brevity.
• Figure 5: Normalized differential distributions of $\Delta\phi_{ee}$ for ALP masses $m_a=5$ GeV (left panel) and $125$ GeV (right panel). The black curves represent the SM background. The red and blue curves correspond to the contribution from the CP-conserving ($aF_{\mu\nu}\tilde{F}^{\mu\nu}$) and CP-violating ($aF_{\mu\nu}F^{\mu\nu}$) ALP-photon interaction. The orange curves denote the coexistence of two equal couplings, i.e.,$\tilde{C}_\gamma=C_\gamma$.