Invariant mass

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

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