Main Page | See live article | Alphabetical index 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 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 This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.