In Yang-Mills theory and specifically in its application to QCD, the theta angle refers to the prefactor $\theta$ in the expression of the action functional of the theory in front of the piece of topological Yang-Mills theory
(see also at S-duality for more).
In phenomenology the theta angle has to be very close to an integer multiple of $\pi$, see at CP problem. That it is indeed $\theta_{QCD}\simeq 0$ instead of $\theta_{QCD} \simeq \pi$ that matches experiment is argued at the end of Crewther-DiVecchia-Veneziano-Witten 79, PO discussion.
Rodney Crewther, Paolo Di Vecchia, Gabriele Veneziano, Edward Witten, Chiral estimate of the electric dipole moment of the neutron in quantum chromodynamics, Phys. Lett. B 88 (1979) 123-127 (CERN). See also
Davide Gaiotto, Anton Kapustin, Zohar Komargodski, Nathan Seiberg, Theta, Time Reversal, and Temperature (arXiv:1703.00501)
Discussion of a similar $\theta$-angle in a 3d field theory, via extended TQFT and stable homotopy theory is in
Last revised on January 3, 2018 at 17:57:07. See the history of this page for a list of all contributions to it.