Nelson-Barr Mechanism

A theoretical framework designed to solve the Strong CP Problem without requiring the axion. It posits that the puzzlingly small value of the QCD θ-angle is not fine-tuned but is dynamically driven to zero by the introduction of new, heavy fermions and a specific pattern of CP violation in a "vector-like" sector of particle physics. In this model, CP is a fundamental symmetry that is spontaneously broken at a high energy scale. This breaking transmits a CP-violating phase to the weak interactions (as observed) but cleverly shields the strong interactions from acquiring a large θ-angle.

CP symmetry might be a true synmetry of nature, but one that is slightly broken in a way that keeps 9 small. In 1984, Ann Nelson and Stephen Barr showed how this could work, but their mechanism requires the existence of a throng of heavy particles that may be too heavy to detect in experiments.

It is an elegant, if more complex, alternative to the axion solution, demonstrating the creative ingenuity physicists apply to solve deep naturalness problems in the Standard Model.