Zeroth law of black hole thermodynamics for higher derivative Proca theories

We prove the constancy of surface gravity across a Killing horizon (not necessarily of bifurcate type) in arbitrary higher curvature theories of gravity coupled to Proca fields $-$ vector fields lacking $U(1)$ gauge invariance $-$ thus generalizing the zeroth law to this broader class of theories. This is achieved within the framework of effective field theory, where higher curvature contributions are treated perturbatively around the leading two derivative theory. The result holds to arbitrary order in the effective field theory expansion. The proof is based on boost-weight arguments; implementing these arguments in the presence of a Proca field introduces subtleties beyond those encountered in the pure-gravity case, which we address here.