Probing ALP couplings to electroweak gauge bosons

Abstract

Motivated by the more and more abundant experimental data, we revisit the couplings of axion-like particle (ALP) to electroweak gauge bosons across the ALP mass range from MeV to 100 GeV. The current and future experimental limits on the couplings are extended. The ALP coupling to $W$-bosons gives rise to flavor-changing ALP-quark couplings at the one-loop level. These flavor-changing couplings deserve further investigation under current experimental constraints, especially those stemming from rare meson decays and neutral meson mixing processes. Additionally, flavor-conserving couplings of the ALP to Standard Model (SM) fermions arise at the one-loop level as well from ALP-electroweak gauge boson couplings, even in the absence of tree-level couplings to these SM fermions, with consequent ALP decays to the SM fermions leading to constraints on the ALP-electroweak gauge boson couplings. We also investigate processes relevant to $Z$-boson measurements, such as the invisible decay $Z\to a\gamma$, subsequent decays $Z\to 3\gamma$ and $Z\to \gamma ll$, as well as constraints from oblique parameters ($S,\, T,\, U$). Our study highlights that rare two-body decays of pseudoscalar mesons offer the most sensitive probes of ALP couplings to electroweak gauge bosons from the loop-induced flavor-violating interactions for ALP masses below the kinematic threshold, while $Z$-boson decays complementarily explore larger ALP masses. Future lepton colliders, such as CEPC and FCC-ee operating at the $Z$-pole, along with SHiP, provide further opportunities to probe ALP couplings to electroweak gauge bosons.