feynlag.pheno.amplitudes.polarization_sum

feynlag.pheno.amplitudes.polarization_sum(p, mass, a, b)

Vector polarization sum Σ ε_a ε*_b = −g_ab + p_a p_b/m².

For mass == 0 the longitudinal piece is absent and the sum reduces to −g_ab: legitimate because every vertex feynlag extracts is contracted with a conserved current, so the gauge-dependent remainder cancels in |M|².