Positivity and partial wave unitarity bounds on ALP theories via amplitude methods

Abstract

We derive the complete set of partial wave unitarity bounds on the most general Axion-Like Particle (ALP) effective interactions up to dimension 8 in the limit of large center-of-mass energy. Exploiting a recently developed formalism based on spinor-helicity techniques, we discuss the unitarity bounds for $N \to M$ (with $N, M \geq 2$) scattering amplitudes that can be relevant for ALP searches at colliders or in a variety of rare processes. Moreover, we compute positivity bounds on ALP interactions, emphasizing their complementarity with partial wave unitarity bounds. As a byproduct, we show that our results can be used to infer new positivity constraints in the Standard Model Effective Field Theory.