 Mandelstam variables

For two-to-two scattering $A+B\to C+D$,They satisfy $s+t+u=m_A^2+m_B^2+m_C^2+m_D^2$ in units $c=1$.

Mandelstam variables are quantities used in particle physics to describe the kinematics of scattering processes. They provide a convenient way to express the conservation laws and relationships between the energies and momenta of the particles involved.