So, as far as I know, the Strong CP Problem in QCD results from the theta angle term in the action: $i\theta\int_X F_\nabla\wedge F_\nabla$ where $\nabla$ is the gauge connection and $X$ is a manifold on which the theory is defined. This term obviously breaks CP symmetry with non-zero choice of theta angle. Correct me if I am wrong.

At any rate, experimental evidence has shown CP symmetry to be a consistent aspect of QCD, and the Strong CP Problem is essentially to discover CP violation or to prove CP symmetry in the lagrangian. Now, I am wondering about the particulars of various solutions to the problem. In particular, is it necessary to fabricate a new particle such as the axion, or are there (hypothetically) simpler and more easily verifiable ways of solving the problem?

Also, how important is the problem? In other words, would making experiment and theory consistent warrant a Nobel Prize? Or is it simply an irritating discrepancy that is not fundamentally important to our understanding of the Standard Model?

This post imported from StackExchange Physics at 2014-11-28 11:30 (UTC), posted by SE-user ciao