The Low-Energy Effective Theory of Axions and ALPs

TL;DR

This paper develops a comprehensive low-energy effective theory for axions and ALPs, detailing their coupling evolution, anomalous dimensions, and interactions with photons and mesons, including flavor-changing effects.

Contribution

It provides the first detailed derivation of ALP coupling evolution from high-energy scales to low energies, including two-loop anomalous dimensions and flavor effects.

Findings

Derived two-loop anomalous dimensions for ALP couplings.

Constructed the effective chiral Lagrangian for ALPs with photons and mesons.

Analyzed flavor-changing effects at the weak scale.

Abstract

Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model, which interact with the known particles through higher-dimensional operators suppressed by the mass scale $\Lambda$ of the new-physics sector. Starting from the most general dimension-5 interactions, we discuss in detail the evolution of the ALP couplings from the new-physics scale to energies at and below the scale of electroweak symmetry breaking. We derive the relevant anomalous dimensions at two-loop order in gauge couplings and one-loop order in Yukawa interactions, carefully considering the treatment of a redundant operator involving an ALP coupling to the Higgs current. We account for one-loop (and partially two-loop) matching contributions at the weak scale, including in particular flavor-changing effects. The relations between different equivalent forms of the effective Lagrangian are discussed in detail. We also construct the effective chiral Lagrangian for an ALP interacting with photons and light pseudoscalar mesons, pointing out important differences with the corresponding Lagrangian for the QCD axion.