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)−14FμνFμν−V(ψ†ψ)
Where V is the scalar potential, Fμν=∂μAν−∂νAμ is the field strength tensor and Dμ=∂μ+ieAμ is the covariant derivative.
Under the U(1) transformation our fields transform infintesimally as:
ψ↦ψ+iα(x)ψ,ψ†↦ψ†−iα(x)ψ†,Aμ↦Aμ−1e∂μα(x)
The Noether current can be found:
jμ=ΠμψDψ+Πμψ†Dψ†+ΠμAνDAν+Wμ
Where Dϕ≡∂ϕ∂α|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(∂μψ†−∂μψ)+e2AμAμψ†ψ−14FμνFμν−V(ψ†ψ)
Thus:
Πμψ=∂L∂(∂μψ)=∂μψ†−ieAμΠμψ†=∂L∂(∂μψ†)=∂μψ+ieAμΠμAν=∂L∂(∂μAν)=−Fμν
And:
Dψ=iψ,Dψ†=−iψ†,DAμ=−1e∂μα
This would give us a Noether current of:
jμ=i(∂μψ†−ieAμ)ψ−i(∂μψ+ieAμ)ψ†+1eFμν∂να
But this seems strange to me, not least because it still contains α.
What have I misunderstood here?
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