Unitarity constraints on ALP interactions

Abstract

We derive partial-wave unitarity constraints on gauge-invariant interactions of an Axion-Like Particle (ALP) up to dimension-6 from all allowed $2\to 2$ scattering processes in the limit of large center-of-mass energy. We find that the strongest bounds stem from scattering amplitudes with one external ALP and only apply to the coupling to a pair of $SU(2{)}_L$ gauge bosons. Couplings to $U(1{)}_Y$ and $SU(3{)}_C$ gauge bosons and to fermions are more loosely constrained.

Comments: 11 pages, 1 figure, 5 tables. Version published in PRD