Ward identity
= Ward identity
{c}
{wiki=Ward–Takahashi_identity}
For an on-shell amplitude $\mathcal M=\epsilon_\mu(k)\mathcal M^\mu$ with an external photon, gauge invariance implies $k_\mu\mathcal M^\mu=0$.
= Ward identity
{c}
{wiki=Ward–Takahashi_identity}
For an on-shell amplitude $\mathcal M=\epsilon_\mu(k)\mathcal M^\mu$ with an external photon, gauge invariance implies $k_\mu\mathcal M^\mu=0$.