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DOE OSTI · 3014756

Clearing up the Strong $CP$ problem

Benabou, Joshua N.·Hook, Anson·Manzari, Claudio Andrea·Murayama, Hitoshi·Safdi, Benjamin R.
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Abstract

The absence of a neutron electric dipole moment (EDM) constrains the quantum chromodynamics (QCD) theta angle to be less than one part in ten billion, posing the Strong $CP$ problem. We revisit two classes of proposed solutions. First, we show that when $P$ or $CP$ is realized as a gauged discrete symmetry - as can arise in quantum gravity - the vacuum necessarily preserves $CP$, contrary to recent claims that discrete-symmetry solutions fail. Gauged discrete models face model-building challenges, such as avoiding contributions to the neutron EDM after spontaneous $P$ or $CP$ breaking, but in principle have no fundamental obstructions. Second, we critically examine recent arguments that the Strong $CP$ problem is illusory, demonstrating that a nonzero neutron EDM at finite $\barθ$ follows directly from well-understood QCD dynamics. Taken together, our results reinforce the reality of the Strong $CP$ problem and highlight gauged discrete-symmetry realizations of $P$ or $CP$ as plausible solutions.

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Benabou, Joshua N., Hook, Anson, Manzari, Claudio Andrea, Murayama, Hitoshi, Safdi, Benjamin R.. 2025-01-01. Clearing up the Strong $CP$ problem. https://doi.org/10.48550/arxiv.2510.18951

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