(In)dependence of 𝜃 in the Higgs regime without axions

Abstract

We revisit the issue of the vacuum angle [Formula: see text] dependence in weakly coupled (Higgsed) Yang–Mills theories. Two most popular mechanisms for eliminating physical [Formula: see text] dependence are massless quarks and axions. Anselm and Johansen noted that the vacuum angle [Formula: see text], associated with the electroweak SU(2) in the Glashow–Weinberg–Salam model (Standard Model, SM), is unobservable although all fermion fields obtain masses through Higgsing and there is no axion. We generalize this idea to a broad class of Higgsed Yang–Mills theories. In the second part, we consider the consequences of Grand Unification. We start from a unifying group, e.g. SU(5), at a high ultraviolet scale and evolve the theory down within the Wilson procedure. If on the way to infrared the unifying group is broken down into a few factors, all factor groups inherit one and the same [Formula: see text] angle — that of the unifying group. We show that embedding the SM in SU(5) drastically changes the Anselm–Johansen conclusion: the electroweak vacuum angle [Formula: see text], equal to [Formula: see text] becomes in principle observable in [Formula: see text] processes. We also note in passing that if the axion mechanism is set up above the unification scale, we have one and the same axion in the electroweak theory and QCD, and their impacts are interdependent.