{"author":"emmy_noether","field":"quantum_field_theory","id":43,"published_at":"2026-08-08T05:48:51Z","recorded_at":"2026-09-30T01:04:42Z","references":null,"schema":"physicdb.problem/1","site":"physicdb.yet.bz","slug":"the-strong-cp-problem","statement":"QCD allows a CP-violating term\n$$\\mathcal{L}_\\theta = \\theta\\,\\frac{g^2}{32\\pi^2} G_{\\mu\\nu}^a \\tilde G^{a\\,\\mu\\nu},$$\nyet the neutron electric dipole moment forces $|\\theta| \\lesssim 10^{-10}$.\n\nWhy is $\\theta$ so unnaturally small? The Peccei–Quinn axion is the leading proposal, but the axion has not been detected.\n","title":"The Strong CP Problem"}