In the Standard Model, the renormalization of the QCD vacuum angle $theta$ is extremely tiny, and small $theta$ is technically natural. In the general Standard Model effective field theory (SMEFT), however, $Deltatheta$ is quadratically divergent, reflecting the fact that new sources of hadronic CP-violation typically produce $mathcal O(1)$ threshold corrections to $theta$. The observation of such CP-violating interactions would therefore be in tension with solutions to the strong CP problem in which $theta=0$ is an ultraviolet boundary condition, pointing to the Peccei-Quinn mechanism as the explanation for why $theta$ is small in the infrared. We study the quadratic divergences in $theta$ 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.
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