Unitarity Constraints On Alp Interactions

We derive partial-wave unitarity constraints on gauge-invariant interactions of an axionlike particle up to dimension-6 from all allowed 2 -> 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 axionlike particle 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.