In theoretical physics, the Mandelstam variables are numerical quantities that encode the energy, momentum, and angles of particles in a scattering process in a Lorentz-invariant fashion. They are used for scattering processes of two particles to two particles. The Mandelstam variables were first introduced by physicist Stanley Mandelstam in 1958.
If the Minkowski metric is chosen to be , the Mandelstam variables  are then defined by
where p and p are the four-momenta of the incoming particles and p and p are the four-momenta of the outgoing particles.
 is also known as the square of the center-of-mass energy (invariant mass) and  as the square of the four-momentum transfer.