Thursday, February 21, 2019

conservation laws - What is the Noether charge associated with the the color SU(3) symmetry of QCD?



A version of the Noether's theorem applies to local gauge symmetries. What is the Noether's charge associated with a non-abelian gauge symmetry such as the color SU(3) and how is that derived? I want an expression for the color charge operator like we have an expression for the electric charge operator. Please see Eq. (9) and (11) of the answer here.



Answer



The SU(3) gauge symmetry is a local symmetry, and therefore it is not Noether's first, but Noether's second theorem that applies to it, which does not yield conserved quantities.


For U(1) gauge symmetries like the electromagnetic symmetry, there is also a global U(1) symmetry, and hence a conserved quantity. But the global symmetry associated to a non-Abelian gauge symmetry is just the center of the gauge group, which is discrete for SU(3), and hence there is no conserved quantity associated to it. This center symmetry has physical significance e.g. in models of confinement, see this question and its answer.


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