Fermion spin sum
= Fermion spin sum
For on-shell Dirac spinors in standard relativistic normalization,
$$
\sum_ru_r(p)\bar u_r(p)=\not p+m,
\qquad
\sum_rv_r(p)\bar v_r(p)=\not p-m.
$$
These completeness identities turn spin-summed squared amplitudes into gamma-matrix traces.
= Fermion spin sums
{synonym}