Topological Yang–Mills theory
In gauge theory, topological Yang–Mills theory, also known as the theta term or -term is a gauge-invariant term which can be added to the action for four-dimensional field theories, first introduced by Edward Witten.[1] It does not change the classical equations of motion, and its effects are only seen at the quantum level, having important consequences for CPT symmetry.[2]
Action
[edit]Spacetime and field content
[edit]The most common setting is on four-dimensional, flat spacetime (Minkowski space).
As a gauge theory, the theory has a gauge symmetry under the action of a gauge group, a Lie group , with associated Lie algebra through the usual correspondence.
The field content is the gauge field , also known in geometry as the connection. It is a -form valued in a Lie algebra .
Action
[edit]In this setting the theta term action is[3] where
- is the field strength tensor, also known in geometry as the curvature tensor. It is defined as , up to some choice of convention: the commutator sometimes appears with a scalar prefactor of or , a coupling constant.
- is the dual field strength, defined .
- is the totally antisymmetric symbol, or alternating tensor. In a more general geometric setting it is the volume form, and the dual field strength is the Hodge dual of the field strength .
- is the theta-angle, a real parameter.
- is an invariant, symmetric bilinear form on . It is denoted as it is often the trace when is under some representation. Concretely, this is often the adjoint representation and in this setting is the Killing form.
As a total derivative
[edit]The action can be written as[3] where is the Chern–Simons 3-form.
Classically, this means the theta term does not contribute to the classical equations of motion.
Properties of the quantum theory
[edit]CP violation
[edit]Chiral anomaly
[edit]See also
[edit]References
[edit]- ^ Witten, Edward (January 1988). "Topological quantum field theory". Communications in Mathematical Physics. 117 (3): 353–386. Bibcode:1988CMaPh.117..353W. doi:10.1007/BF01223371. ISSN 0010-3616. S2CID 43230714.
- ^ Gaiotto, Davide; Kapustin, Anton; Komargodski, Zohar; Seiberg, Nathan (17 May 2017). "Theta, time reversal and temperature". Journal of High Energy Physics. 2017 (5): 91. arXiv:1703.00501. Bibcode:2017JHEP...05..091G. doi:10.1007/JHEP05(2017)091. S2CID 256038181.
- ^ a b Tong, David. "Lectures on gauge theory" (PDF). Lectures on Theoretical Physics. Retrieved August 7, 2022.