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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 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