Tuesday, October 18, 2016

quantum field theory - Applying Noether's Theorem to local invariance


I have realised that I am unsure about how I can apply Noether's theorem to a Lagrangian with local invariance. For instance, the following Lagrangian has a local U(1) invariance:


L=(Dμψ)(Dμψ)14FμνFμνV(ψψ)


Where V is the scalar potential, Fμν=μAννAμ is the field strength tensor and Dμ=μ+ieAμ is the covariant derivative.


Under the U(1) transformation our fields transform infintesimally as:


ψψ+iα(x)ψ,ψψiα(x)ψ,AμAμ1eμα(x)



The Noether current can be found:


jμ=ΠμψDψ+ΠμψDψ+ΠμAνDAν+Wμ


Where Dϕϕα|α=0. And where DL=μWμ. So we can see that Wμ=0. Using our Lagrangian and expanding the covariant derivatives, we have:


L=(μψ)(μψ)+ieAμ(μψμψ)+e2AμAμψψ14FμνFμνV(ψψ)


Thus:


Πμψ=L(μψ)=μψieAμΠμψ=L(μψ)=μψ+ieAμΠμAν=L(μAν)=Fμν


And:


Dψ=iψ,Dψ=iψ,DAμ=1eμα


This would give us a Noether current of:


jμ=i(μψieAμ)ψi(μψ+ieAμ)ψ+1eFμννα



But this seems strange to me, not least because it still contains α.


What have I misunderstood here?




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