Indirect signs of the Peccei-Quinn mechanism.

In the standard model, the renormalization of the QCD vacuum angle θ is extremely tiny, and small θ is technically natural. In the general standard model effective field theory (SMEFT), however, Δθ is quadratically divergent, reflecting the fact that new sources of hadronic CP-violation typically produce O (1) threshold corrections to θ. The observation of such CP-violating interactions would therefore be in tension with solutions to the strong CP problem in which θ = 0 is an ultraviolet boundary condition, pointing to the Peccei-Quinn mechanism as the explanation for why θ is small in the infrared. We study the quadratic divergences in θ arising from dimension-6 SMEFT operators and discuss the discovery prospects for these operators at electric dipole moment experiments, the LHC, and future proton-proton colliders. [ABSTRACT FROM AUTHOR]