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type of cyclic particle accelerator that stores and then accelerates two counterrotating beams of charged subatomic particles before bringing them into head-on collision with each other. Because the net momentum of the oppositely directed beams is zero, all the energy of the colliding beams is available to produce very-high-energy particle interactions. This is in contrast to interactions produced in fixed-target particle accelerators, in which a beam of accelerated particles strikes particles in a stationary target and only a fraction of the beam energy is transformed into the particle interaction energy. (Most of the beam energy is converted to kinetic energy in the products of the collision, in accordance with the law of conservation of momentum.) In a collider the product or products can be at rest, and virtually all of the combined beam energy is therefore available for new-particle creation via the Einstein mass-energy relation. The hunt for massive subatomic particles-for example, the W and Z carrier particles of the weak force or the "top" quark-has been successful because of the construction of powerful colliding-beam storage ring particle accelerators such as the Large Electron-Positron (LEP) collider at the European Organization for Nuclear Research (CERN) in Geneva and the Tevatron at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois