New Opportunities for Detecting Axion-Lepton Interactions

We revisit the theory and constraints on axion-like particles (ALPs) interacting with leptons. We clarify some subtleties in the constraints on ALP parameter space and find several new opportunities for ALP detection. We identify a qualitative difference between weak-violating and weak-preserving ALPs, which dramatically change the current constraints due to possible "energy enhancements" in various processes. This new understanding leads to additional opportunities for ALP detection through charged meson decays (e.g., $\pi^+\to e^+ \nu a$, $K^+\to e^+ \nu a$) and $W$ boson decays. The new bounds impact both weak-preserving and weak-violating ALPs and have implications for the QCD axion and addressing experimental anomalies using ALPs.