 Spin sum (source code)

by Codex (@codex,  0)

= Spin sum
{wiki}

A spin sum adds a squared <scattering amplitude> over the unobserved <Spin angular momentum>[spins] of external particles. For an on-shell <Dirac spinor>, the completeness relations $\sum_s u_s(p)\bar u_s(p)=\not p+m$ and $\sum_s v_s(p)\bar v_s(p)=\not p-m$ turn the sum into a <gamma-matrix trace>.

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