Colliders

Einstein's famous equation E=mc² tells us that energy and mass are equivalent. Thus the energy of a particle beam can convert into mass, creating a fascinating wealth of additional particles, many of them highly unstable and not normally found in nature. However if the incoming beam is simply slammed into a stationary target, much of the projectile energy is taken up by the target's recoil and not exploitable. Much more energy is available for the production of new particles if two beams travelling in opposite directions are collided together. All the world's major particle physics projects now under way concentrate on such colliding beam machines.

The electron positron accelerator LEP is a collider. Its 3368 magnets bend two particle beams and keep them on orbit. Where negatively charged electrons bend one way, positively charged positrons bend the other. This allows LEP to circulate 90 GeV (*) beams of electrons and positrons in opposite directions using the same magnets. An other CERN collider, the Super Proton Synchrotron (SPS), uses the same technique to circulate protons in one direction and anti-protons in the opposite direction.

* Mass and Energy

A GeV, or giga electron volt, to give it its full name, is a unit of energy used by particle physicists, it is also used as a unit of mass. Mass (m) is just a form of energy (E), as Einstein showed in his famous equation E=mc², c is a constant (the speed of light). When high energy particle beams collide, new particles are formed as energy is converted into matter. A proton corresponds to an energy of a little less than 1 GeV.