Abstract
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, $\Delta\theta$ 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.
Highlights
The absence of an electric dipole moment (EDM) for the neutron strongly constrains the CP-violating QCD vacuum angle to be tiny, θ ≲ 10−10 [1,2,3,4]
The top chromo-EDM is of particular interest, and we study the potential for the high-luminosity 14 TeV LHC and future 27 and 100 TeV colliders to detect the top cEDM in simple angular observables, where it can be distinguished from a CP-conserving magnetic moment
Searches for new sources of CP violation beyond the standard model are of fundamental importance, probing symmetry structure and the origin of the matter-antimatter asymmetry
Summary
The absence of an electric dipole moment (EDM) for the neutron strongly constrains the CP-violating QCD vacuum angle to be tiny, θ ≲ 10−10 [1,2,3,4]. In models with extra stronglycoupled gauge sectors coupled to the SM, new vacuum angles can shift θ at the confinement scale of the new sector [35,36] These corrections generically spoil UV solutions to strong CP.. If integrating out heavy fields generates CPV SMEFT operators involving quarks and gluons, it will typically produce threshold corrections to θ These corrections are not calculable in the low energy theory, but traces of them remain, including quadratically divergent corrections to θ involving the effective operators. Cutoff-independent, Λ2=Λ2, and the relevant UV physics includes any new sources of CPV coupled to quarks or gluons Evidence for these operators sharpens the unnaturalness of small θ and strongly disfavors the possibility of natural UV solutions.
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