nLab
theta angle

Contents

Idea

In Yang-Mills theory and specifically in its application to QCD, the theta angle refers to the prefactor θ\theta in the expression of the action functional of the theory in front of the piece of topological Yang-Mills theory

1g 2 XF F +iθ XF F \nabla \mapsto \frac{1}{g^2 }\int_X F_\nabla \wedge \star F_\nabla \;+\; i \theta \int_X F_\nabla \wedge F_\nabla

(see also at S-duality for more).

In phenomenology the theta angle has to be very close to an integer multiple of π\pi, see at CP problem. That it is indeed θ QCD0\theta_{QCD}\simeq 0 instead of θ QCDπ\theta_{QCD} \simeq \pi that matches experiment is argued at the end of Crewther-DiVecchia-Veneziano-Witten 79, PO discussion.

References

Discussion of a similar θ\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.