Peccei-Quinn Mechanism

An elegant and influential theoretical solution to the Strong CP Problem. It proposes a new, global symmetry (the Peccei-Quinn symmetry) that is spontaneously broken at a very high energy scale. As a consequence of this breaking, a new, very light, pseudo-scalar particle is predicted: the axion. This axion field dynamically relaxes the offending CP-violating parameter (θ) in QCD to zero, solving the fine-tuning problem naturally.

In 1977, Roberto Peccei and Helen Quinn proposed that 6 is not a constant, but a field whose value can evolve. The approximate symmetries of the field cause the 6 parameter to become small or zero. Quantum fields always have associated particles, and so the field of the Peccei-Quinn mechanism predicts a new particle: the "aidon," which has come in handy as a theoretical solution for other cosmic mysteries. This is the most popular solution to the strong CP problem; though there's no experimental evidence for it yet, searches for the axion are in progress.

The mechanism is brilliant because it turns a puzzle into a prediction: the axion, originally postulated to solve a symmetry problem, is now a prime, well-motivated candidate for cold dark matter. Thus, this single, beautiful idea addresses two of the greatest mysteries in physics—the nature of the strong force and the identity of the invisible matter that shapes the cosmos.