You often see in textbooks the statement that Tμμ=0 implies Weyl invariance or conformal invariance. The proof goes like
δS∼∫√gTμνδgμν∼∫√gTμμ,
where I have varied the action with respect to the metric and assumed δgμν∝gμν (i.e. a Weyl transformation).
This does not seem to be completely general because I can imagine a Lagrangian containing matter fields with non-trivial conformal weights. Then the full variation under Weyl tranformation contains a term proprotional to the matter equation of motion.
So I would conclude that the correct statement is more like
Tμμ=0,&δSδϕ=0⟹Weyl invariant
Is it true that Weyl invariance only holds when the matter fields are on-shell or am I missing something?
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