CP Violation & the Strong CP Problem
Mirror symmetry (CP) is exact in the fundamental law. But when we build heavier particles (strange, charm, bottom, top quarks), the vacuum has two almost identical ground states that are mirror images of each other — like two valleys separated by a tiny ridge.
Our universe, for reasons we do not yet know, fell into one valley and not the other. That single choice billions of years ago explains every single CP-violating decay we see in particle physics experiments, and simultaneously forces the dangerous “strong CP angle” to be smaller than 10⁻¹⁰ — solving the strong CP problem without axions, extra dimensions, or any new particles.
Spontaneous vacuum selection induces an effective topological term
\[ \mathcal{L}_\text{eff} \supset \theta_\text{eff} \frac{g^2}{32\pi^2} F\widetilde{F}, \quad \theta_\text{eff} \simeq 10^{-10}\text{--}10^{-11} \]
exactly the size needed.