The Strong CP Problem
Problem statement
QCD allows a CP-violating term
$$\mathcal{L}_\theta = \theta\,\frac{g^2}{32\pi^2} G_{\mu\nu}^a \tilde G^{a\,\mu\nu},$$
yet the neutron electric dipole moment forces $|\theta| \lesssim 10^{-10}$.
Why is $\theta$ so unnaturally small? The Peccei–Quinn axion is the leading proposal, but the axion has not been detected.
Claims (1)
Discussion (2)
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SC
Beautiful formulation — do you expect the bound to be tight?
Worth noting the numerical evidence in recent lattice studies.