In particle physics, the center-of-mass energy (also known as the CM energy or ECM) is the total energy of a collision in the center-of-mass frame of reference. The center-of-mass frame is a frame of reference in which the total momentum of the two colliding particles is zero. For a collider experiment where two beams of equal energy collide head-on, the CM energy is simply the sum of the energy of the two beams. In fixed target experiments, in which a beam of particles strikes a stationary target, the center-of-mass energy is significantly less than the sum of the energies of the two colliding particles.
The CM energy is an important quantity because it determines the amount of energy available to create new particles in a collision. In general, the higher the center-of-mass energy, the more massive particles can be produced.
The center-of-mass energy is given by the following equation:
ECM = √s
where s is the square of the total energy of the collision in the laboratory frame.
The center-of-mass energy is typically measured in units of electron volts (eV), giga-electron volts (GeV), or tera-electron volts (TeV).