feynlag.pheno.propagator.breit_wigner

feynlag.pheno.propagator.breit_wigner(q2, mass, width)

The squared-modulus denominator 1/((q²−m²)² + m²Γ²).

This is $|1/(q^2 - m^2 + i mGamma)|^2$ — the Breit–Wigner factor that turns a would-be on-shell pole into the finite off-shell line shape. The resonance width Γ is itself a decay-calculator output (self-referential: the $W/Z$ width feeding the propagator comes from Tier-1).