2025/10/21 by Joshua N. Benabou, Benabou, Joshua N., Anson Hook +7 · 3 citations
Physics and Astronomy · #FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph) #High Energy Physics - Theory (hep-th) #Noncommutative and Quantum Gravity Theories #Particle physics theoretical and experimental studies #Quantum Chromodynamics and Particle Interactions
paper · pdf · doi:10.48550/arxiv.2510.18951
openalex publication_date 2025/10/21 · openalex created_date 2025/10/24 · openalex updated_date 2026/08/01
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 θ 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.