Canonical Quantization of Scalar Fields — Mark Srednicki
Physics — QFT · ward-identity-scalar-qed-compton
Problem
Show explicitly that the tree-level e~+e~−→γγ scattering amplitude in scalar electrodynamics,
T=−e2[m2−t4(k1⋅ε1′)(k2⋅ε2′)+m2−u4(k1⋅ε2′)(k2⋅ε1′)+2(ε1′⋅ε2′)],
vanishes if ε1′μ is replaced with k1′μ.