Boltzmann scattering kernel ($B(g,\theta )$)

The nonnegative weight describing the rate and angular dependence of binary elastic collisions in the Boltzmann equation. In a scattering-angle representation with solid-angle measure $d\Omega$, it contains relative speed $g$ times differential scattering cross-section. A parametrization by collision normals has a different Jacobian. Any symmetry factor from integrating over a hemisphere instead of a sphere must be included consistently.