Invariant flux factor ($\mathcal{F}=4\sqrt{(p\cdot q{)}^2-m_1^2{m}_2^2}$)

For two incident particles in relativistic normalization, this is $2\sqrt{\kappa (s,m_1^2,m_2^2)}$, where $\kappa$ is the Källén function. For equal masses it is $2\sqrt{s(s-4m^2)}$.