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v3.3.2
The experimental tests of the strong interactions have not provided any hint of CP violation in QCD-mediated processes. The prototypical CP-odd observable is the electric dipole moment of the neutron, for which there are stringent bounds. However, the QCD Lagrangian admits a CP-odd topological term proportional to the so-called theta angle. The generic expectation is that such a CP-odd interaction should lead to a nonzero neutron dipole moment, unless there is a severe tuning of the theta angle against the phases of the quark masses. This need of tuning to explain experimental observations constitutes the strong CP problem. In this talk, we challenge the conventional view by presenting novel calculations using either path integral or canonical quantization methods that lead to the conclusion that the theta angle drops out of correlators and becomes thus unobservable, so that the CP symmetry is automatically preserved in the strong interactions without the need of tuning.