Does anyone know a simple explanation for the following statement:
Gauge invariance ⇒ $Πμνϒϒ(0) = ΠμνϒZ(0) = 0$
Where ΠVV' is the V to V' one loop correction, ϒ is the photon field and Z is the Z-boson field. The argument of Π is the incoming momentum q2 = 0