Källén function
= Källén function
{c}
{title2=$\kappa(x,y,z)=x^2+y^2+z^2-2xy-2xz-2yz$}
The Källén function controls two-body relativistic kinematics. In the rest frame of total invariant mass $\sqrt s$, the magnitude of each final momentum is $\sqrt{\kappa(s,m_1^2,m_2^2)}/(2\sqrt s)$. It also determines the <invariant flux factor> in a two-particle collision.