A collider is a type of a particle accelerator involving directed beams of particles. Colliders may either be ring accelerators or linear accelerators, and may collide a single beam of particles against a stationary target or two beams head-on.
Colliders are used as a research tool in particle physics by accelerating elementary particles to very high kinetic energy and letting them impact other particles. Analysis of the byproducts of these collisions gives scientists good evidence of the structure of the subatomic world and the laws of nature governing it. These may become apparent only at high energies and for tiny periods of time, and therefore may be hard or impossible to study in other ways.
In particle physics one gains knowledge about elementary particles by accelerating particles to very high kinetic energy and letting them impact on other particles. For sufficiently high energy, a reaction happens that transforms the particles into other particles. Detecting these products gives insight into the physics involved.
To do such experiments there are two possible setups:
- Fixed target setup: A beam of particles (the projectiles) is accelerated with a particle accelerator, and as collision partner, one puts a stationary target into the path of the beam.
- Collider: Two beams of particles are accelerated and the beams are directed against each other, so that the particles collide while flying in opposite directions. This process can be used to make strange and anti-matter.
The collider setup is harder to construct but has the great advantage that according to special relativity the energy of an inelastic collision between two particles approaching each other with a given velocity is not just 4 times as high as in the case of one particle resting (as it would be in non-relativistic physics); it can be orders of magnitude higher if the collision velocity is near the speed of light.
The first serious proposal for a collider originated with a group at the Midwestern Universities Research Association (MURA). This group proposed building two tangent radial-sector FFAG accelerator rings. Tihiro Ohkawa, one of the authors of the first paper, went on to develop a radial-sector FFAG accelerator design that could accelerate two counterrotating particle beams within a single ring of magnets. The third FFAG prototype built by the MURA group was a 50 MeV electron machine built in 1961 to demonstrate the feasibility of this concept.
Gerard K. O'Neill proposed using a single accelerator to inject particles into a pair of tangent storage rings. As in the original MURA proposal, collisions would occur in the tangent section. The benefit of storage rings is that the storage ring can accumulate a high beam flux from an injection accelerator that achieves a much lower flux.
The first electron-positron colliders were built in Italy, at the Istituto Nazionale di Fisica Nucleare in Frascati near Rome, by the Austrian-Italian physicist Bruno Touschek. Around the same time, in the early 1960s, the VEP-1 electron-electron collider was independently developed and built under supervision of Gersh Budker in the Soviet Institute of Nuclear Physics.
In 1966, work began on the Intersecting Storage Rings at CERN, and in 1971, this collider was operational. The ISR was a pair of storage rings that accumulated particles injected by the CERN Proton Synchrotron. This was the first hadron collider, as all of the earlier efforts had worked with electrons or with electrons and positrons.
See also 
- Large Electron–Positron Collider
- Large Hadron Collider
- Very Large Hadron Collider
- Relativistic Heavy Ion Collider
- International Linear Collider
- Storage ring
- Kerst, D. W.; Cole, F. T.; Crane, H. R.; Jones, L. W.; et al. (1956). "Attainment of Very High Energy by Means of Intersecting Beams of Particles". Physical Review 102 (2): 590–591. doi:10.1103/PhysRev.102.590.
- US patent 2890348, Tihiro Ohkawa, "Particle Accelerator", issued 1959-06-09
- Science: Physics & Fantasy, Time, Monday, Feb. 11, 1957.
- O'Neill, G. (1956). "Storage-Ring Synchrotron: Device for High-Energy Physics Research". Physical Review 102 (5): 1418–1419. doi:10.1103/PhysRev.102.1418.
- Kjell Johnsen, The ISR in the time of Jentschke, CERN Courier, June 1, 2003.