{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"ca46b22211f8f860725913622c696aaa4e3e3a446a43b9c396e641783523fede","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["abde15c8ce0348cfcddbcfced9cdb9e83db5f20569ff8b484d8e25d5d199f364","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"d19d8ecc25e60b534e3aa6a8867e1aabbc8262b82889d6913def61d02a504dd9","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":"2","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 $10^{-10}$. No reason for this is known.","posed_since":"","precise":"The physical parameter $\\theta_{\\mathrm{bar}} = \\theta - \\arg \\det(M_{q})$ induces $d_{n} \\sim 10^{-16}\\ \\theta_{\\mathrm{bar}}\\ e\\ \\mathrm{cm}$; the PSI bound $\\mid d_{n}\\mid < 1.8 \\times 10^{-26}\\ e\\ \\mathrm{cm}$ (90 percent CL, 2020) gives $\\mid\\theta_{\\mathrm{bar}}\\mid < \\sim 10^{-10}$. 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":"abde15c8ce0348cfcddbcfced9cdb9e83db5f20569ff8b484d8e25d5d199f364","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 $10^{-10}$?","topic_ref":null},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"14e0a7a855fcba4052b6154b112dfaa3ef24889e1fb767130f0e1ea38a526feb","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"4bd371766082e04b1caf58996a1facb2c1ec2c71d8550bc957ff76439c4dd653","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"fqLdnZX1IuqOmp4bwoKSYBdOGtlh9UNm0B0g-0Vf_4mcLL9sYkeRhQakG9IODyluxUwFKYv44EztgdpchbTpAA"},"schema":"pubphys.envelope/1"},"record_hash":"14e0a7a855fcba4052b6154b112dfaa3ef24889e1fb767130f0e1ea38a526feb","leaf_index":3388}