Leptophilic ALPs in Laboratory Experiments

TL;DR

Leptophilic ALPs, which couple predominantly to charged leptons, acquire loop-induced photon couplings that depend on the process kinematics and differ between derivative and pseudoscalar bases. The paper systematically develops the momentum-dependent ALP-photon vertex, computes production and decay rates, and reinterprets LEP, E137, and NA64 data to set state-of-the-art bounds for four coupling scenarios (electron-, muon-, tau-, and universal-lepton couplings) in the $MeV$--$GeV$ mass range. It also surveys complementary constraints from astrophysics, flavor, and other lab experiments, highlighting the crucial role of the Primakoff and dark-bremsstrahlung channels and the importance of momentum dependence in the effective photon coupling. The results reveal that E137 and NA64, especially with future NA64$$ muon data, significantly constrain leptophilic ALPs, while regions remain open at higher masses and for pseudoscalar couplings, underscoring the need for continued pursuit with Belle II, SHiP, and forward detectors. Overall, the work provides a comprehensive, momentum-aware map of leptophilic ALP parameter space and clarifies how derivative and pseudoscalar implementations lead to distinct experimental signatures.

Abstract

We study the collider phenomenology of leptophilic axion-like particles (ALPs), i.e. pseudoscalar particles that couple only to charged leptons. Loops of charged leptons induce effective interactions of the ALPs with photons, which depend on the momenta of the interacting particles and differ between pseudoscalar and derivative lepton couplings. We systematically discuss the form of the interaction with photons for general external momenta and identify the regimes when it can be safely approximated by an effective coupling constant. We use these results to derive novel constraints from LEP and calculate state-of-the-art limits from E137 and NA64 for four different scenarios, in which the ALPs couple either to a single lepton generation or universally to all, for both pseudoscalar and derivative lepton couplings. We collect complementary bounds from astrophysics, flavour, and other laboratory experiments to chart the allowed parameter space of leptophilic ALPs in the MeV-GeV mass range.

Figures (15)

• Figure 1: Triangle diagram inducing the effective ALP-photon interaction.
• Figure 2: Schematic illustration of a beam dump. A BSM particle $X$ is produced at the target and passes through the dump. It can be detected if it decays into SM particles in the decay volume.
• Figure 3: Integrated electron/positron energy distributions for E137.
• Figure 4: Top row: Feynman diagrams contributing to the process $e N \rightarrow e N a$. The diagrams correspond from left to right to the DB $s$-channel process, the DB $u$-channel process and the Primakoff process. Bottom row: Corresponding Feynman diagrams contributing to the process $e \gamma \rightarrow e a$. Note that the lepton loop has been replaced with the effective Primakoff coupling.
• Figure 5: Normalised cross sections for non-vanishing ALP coupling to electrons as a function of the electron energy $E$. The ALP mass is $m_a = 10MeV$ on the left, $m_a = 450MeV$ on the right. The solid lines represent derivative couplings, while the dashed lines represent pseudoscalar couplings.