Invariant mass

Invariant mass is the mass of a system as determined by its total energy and momentum. It is unchanged by inertial-frame transformations and equals the system’s energy divided by the speed of light squared in its center-of-momentum frame.

Four-momentum: The four-vector combining an object's energy and three-momentum in special relativity. Its Minkowski norm is invariant and determines the system’s invariant mass.
Rest mass: The invariant mass of a single particle, equal to its mass measured in its rest frame. For one particle, invariant mass is its rest mass.
Invariant-mass method: A particle-physics technique for identifying decays by calculating the invariant mass of their detected products. A peak in the products’ invariant-mass distribution can reveal a parent particle.
Relativistic mass: A historically used, frame-dependent quantity defined as an object's total energy divided by c². Unlike invariant mass, it changes with the observer’s frame.
Energy–momentum relation: The relativistic relation connecting energy, momentum, and rest mass: E² = p²c² + m²c⁴. Applied to total system energy and momentum, it gives the invariant mass.
Relativistic energy: The energy assigned to an object in special relativity, including its rest energy and motion-dependent energy. A system’s total energy contributes to its invariant mass together with total momentum.
Particle decay: The transformation of an unstable particle into other particles through a physical interaction. The decay products’ combined invariant mass can identify the decaying particle.
Invariant mass versus rest mass: The distinction between a system’s invariant mass and the rest mass assigned to an individual particle. The terms coincide for a single particle but can obscure the energy of a multiparticle system.
Center-of-momentum frame: A reference frame in which a system's total momentum is zero. In this frame, invariant mass equals total energy divided by c².
Relativistic momentum: Momentum defined in special relativity as the spatial part of four-momentum. Total momentum must be included when extracting invariant mass from a frame’s measurements.

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