Strong CP problem and the QCD axion

4 problems

• How is the Peccei-Quinn symmetry protected from gravitational breaking?

  Open

  The axion solution needs a symmetry to be almost exact, but gravity is expected to break all such global symmetries slightly. Even a tiny breaking would spoil the solution.

• Does the QCD axion exist?

  Open

  The axion would be a very light, very weakly interacting particle that removes the strong CP puzzle. Detectors search for it turning into a photon inside strong magnets.

• Does a nonzero $\theta$ produce observable CP violation in QCD?

  Contested

  A few theorists argue that, done carefully, the strong-force theory never produces the forbidden effects, so there is no puzzle. Most others say the standard reasoning holds, and the disagreement has not been settled.

• Why is the QCD $\theta$ angle smaller than $1e-10$?

  Open

  The strong-force angle $\theta$ could have any value up to about 3, yet the neutron's electric dipole moment shows it is below $1e-10$. No reason for this is known.