Large Hadron Collider

Particle accelerator being constructed at CERN, the European Organization for Nuclear Research, due to begin experiments in 2008. It occupies a 27-kilometre-long ring-shaped tunnel straddling the French-Swiss border near Geneva. The tunnel originally housed the LEP (Large Electron-Positron Collider), now decommissioned.

A major class of LHC experiments will involve colliding protons, the positively charged particles found in the nuclei of all atoms. Physicists hope that they will detect signs of the long-sought Higgs boson among the spray of fragments produced in these events. They will also search for signs of the hidden dimensions predicted by string theory and for companion particles to the familiar subatomic particles, as predicted by supersymmetry. They will scrutinize many other aspects of the present Standard Model of elementary-particle interactions. Other experiments that will be conducted at the LHC include colliding heavy ions (atoms, such as atoms of lead that have lost some of their electrons).

The LHC will accelerate subatomic particles to higher energies than any machine has so far done, guiding the beams with superconducting magnets, and will smash them together in head-on collisions.

Particles will be accelerated to lower energies by a chain of smaller accelerators, and then stored in the tunnel, possibly for hours, circulating in opposite directions in two pipes. These pipes intersect at four locations, where the contrarotating streams of particles will be allowed to collide 40 million times per second. Each proton will have an energy of 7 TeV (7 teraelectronvolts, or 7 trillion electronvolts; a trillion is a million million).