In particle physics, the mathematical combination of a particle's energy and its momentum to give a value for the mass of the particle at rest. The invariant mass is the same for all frames of reference (see Special Relativity).

The invariant mass of a system of decay particles is related to the rest mass of the original particle by the following equation:

W²c⁴=(ΣE)²-(Σpc)²

where:

• W is the invariant mass of the system of particles
• ΣE is the sum of the energies of the particles
• Σp is the vector sum of the momenta of the particles (includes both magnitude and direction of the momenta)
• c is the speed of light