I would like to understand the distinction between an axial anomaly in QCD (Theta Vacuum: axion -> 2 gluons) and an axial anomaly in QCD of current (Chern–Simons term: pion->two photons, photon->three pions, ...). A more specific question: is the current axial anomaly related to the topological properties of the theory like "internal" axial anomaly?
If you don't understand question there are clarifications about "internal" axial anomaly and current axial anomaly (as I see it):
1) First, in quantum chromodynamics, a violation of the axial group UA(1) leads to a nonconservation of the axial current: ∂μJ5,μ=2iˉqˆmqγ5q+Nfg28π2ϵμναβtr(GμνGαβ) ,
where Gμν - gluon field strength tensor. The violation of the axial group is connected with the fact that the vacuum of quantum chromodynamics has a complex topological structure, and this eventually leads to an additional term in the Lagrangian: Lθ=θg216π2ϵμναβtr(GμνGαβ)
2) Second, in addition to the "internal", anomaly of chromodynamics written above, there are external anomalies in the chromodynamics of external currents, the simplest of which corresponds to the process π0→γγ: ∂μJem5,μ=2m(ˉqγ5τ3q)+e216π2ϵμναβFμνFαβ ,
I think this violation is not related to the topological properties of the theory.
In addition to this anomaly, there is a huge number of others, for example an anomaly corresponding to the process γ→πππ. In order to describe all the anomalies, the Wess-Zumino-Witten action is used. This is possible due to the following statement: any non-Abelian anomaly in four-dimensionality can be represented through the action of Wess-Zumino-Witten in five-dimension (Chern-Simons term) (for further information please refer to Can the effective vertex for γ→3π be derived directly from the anomaly?, Chiral anomaly in odd spacetime dimensions). W=−iNc96π2∫10dx5∫d4xϵμνσλρTr[−j−μFLνσFLλρ−j+μFRνσFRλρ−12j+μFLνσU(x5)FRλρU†(x5)−12j+μFRνσU†(x5)FLλρU(x5)+iFLμνj−σj−λj−ρ+iFRμνj+σj+λj+ρ+25j−μj−νj−σj−λj−ρ]
No comments:
Post a Comment