We revisit the issue of the vacuum angle dependence in weakly coupled (Higgsed) Yang-Mills theories. Two most popular mechanisms for eliminating physical dependence are massless quarks and axions. Anselm and Johansen noted that the vacuum angle EW, 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 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 EW, equal to QCD becomes in principle observable in ΔB = ΔL = ±1 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.
Bibliographical noteFunding Information:
We are grateful to Z. Berezhiani, G. Dvali and V. Rubakov for the discussions pertinent to Sec. 4. The work of M.S. was supported in part by DOE Grant No. DE-FG02-94ER40823. A.V. appreciates the hospitality of the Kavli Institute for Theoretical Physics where his research was supported in part by the National Science Foundation under Grant No. NSF PHY-1125915.
- Higgs regime
- Vacuum angle