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