Kavli Affiliate: Hitoshi Murayama
| First 5 Authors: Joshua N. Benabou, Joshua N. Benabou, , ,
| Summary:
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 $bartheta$ 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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