{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"080a87d67b692b7ad61b109097aace078694b0f48298ba1b0174e43b5e761bdc","created":"2026-10-03T07:18:05Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"a4c1e99673a11b78373aa6dc10d48fff34df6fd69b326cb256716e5a20f789d2","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"hep.strong-cp.theta-smallness","kind":"phenomenon","literature_status":"open","n":"1","parents":[],"plain":"The strong-force angle $\\theta$ could have any value up to about 3, yet the neutron's electric dipole moment shows it is below $1e-10$. No reason for this is known.","posed_since":"","precise":"The physical parameter $\\theta_{\\mathrm{bar}} = \\theta - \\arg \\det(M_{q})$ induces $d_{n} \\sim 1e-16\\ \\theta_{\\mathrm{bar}}\\ e\\ \\mathrm{cm}$; the PSI bound $\\mid d_{n}\\mid < 1.8e-26\\ e\\ \\mathrm{cm}$ (90 percent CL, 2020) gives $\\mid\\theta_{\\mathrm{bar}}\\mid < \\sim 1e-10$. An answer is a mechanism (Peccei-Quinn axion, massless up quark, spontaneous CP or P violation such as Nelson-Barr) consistent with lattice QCD and flavor data.","problem_ref":null,"references":"","settled_by":"Detection of the QCD axion with the predicted mass-coupling relation, or a confirmed alternative such as a nonzero measured $d_n$ with the pattern predicted by a Nelson-Barr model.","status_note":"Lattice QCD (2010s to 2020s) excludes $m_{u} = 0$, removing the massless up quark solution.","title":"Why is the QCD $\\theta$ angle smaller than 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