Invariant flux factor
= Invariant flux factor
{title2=$\mathcal F=4\sqrt{(p\cdot q)^2-m_1^2m_2^2}$}
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 $2\sqrt{s(s-4m^2)}$.