Running ALPs across scales : effective field theories and phenomenology of axion-like particles

Abstract

The Standard Model of particle physics (SM) is an astonishingly successful theory that describes nature with almost inconceivable accuracy. Yet, it is incomplete, as several hints point toward the existence of additional particles outside its field content. One of the most promising candidate for such a new state is the axion-like particle (ALP), a generalization of the axion, initially proposed to solve the strong CP problem. This thesis explores ALP effective theories and their phenomenological implications across different energy scales, divided into four parts. The first part is based on the ALP–SMEFT interference, where one-loop ALP exchange generates dimension-six SMEFT operators. By running these effects down to experimental scales and using existing bounds on SMEFT Wilson coefficients, strong model-independent constraints on ALP–SM couplings are derived that are compatible with or even surpass direct searches. In the second part, this concept is systematically extended to the low-energy effective field theory (LEFT), showing how ALPs can contribute to the (g−2)µ via the ALP–LEFT interference. Moving down to even lower energies at and below the scale of chiral symmetry breaking, a consistent ALP extension of chiral perturbation theory at next-to-leading order is constructed. Applying this framework to the flavor-violating decays K±→π±a provides the strongest constraints on ALP–SM couplings for ma ≲300 MeV. Finally, by running the modified RG equation of the quartic Higgs coupling up to the Planck scale, ALP influences on the electroweak stability are analyzed, as well as a possible gauge coupling unification scenario in the presence of this new particle.