Particles and Accelerators Victor Hess discovered in 1912 that the Earth is being bombarded constantly by particles from space, the cosmic rays. Many new types of fundamental particle were discovered among the cosmic rays, including antimatter, the muon, the pion, kaons and other strange particles. Ernest Lawrence and others realised that, in order to study these new particles systematically, physicists had to accelerate particles in their laboratories and examine their collisions. Many new particles were found using these particle accelerators, which enabled them to be understood in a well-tested Standard Model. A comprehensive introduction to particle physics and accelerators is available at CERN website. Quarks and leptons Matter particles can be grouped into two classes, those that have strong nuclear forces, and those that do not. The former are composed of point-like elementary particles called quarks, of which six different types exist in the Universe. The particles without strong nuclear forces are called leptons, and also come in six varieties, including the electron, muon and a heavier but otherwise similar electrically-charged particle (the tau), each of which is accompanied by its own type of neutrino. Four forces In addition to electromagnetism, gravity and the strong nuclear forces, there are also weak nuclear forces responsible for radioactivity. Each of these forces is due to the exchange of other particles, the photon for electromagnetism, the graviton (not yet seen) for gravity, the gluon for the strong nuclear forces, and the W and Z particles for the weak nuclear forces. The Standard Model describing these forces has been verified and tested in many experiments using particle accelerators around the world, in particular the LEP accelerator at CERN, enabling physicists to describe with confidence the history of the Universe since it was about a tenth of a thousandth of a millionth of a second (10-10 sec.) old. For example, experiments at CERN established that there exist just three different types of neutrino, making possible accurate calculations of the amounts of light elements created in the first seconds of the history of the Universe. Experiments Present and future experiments at particle accelerators seek new physics beyond the Standard Model. They search for particles that might constitute the dark matter in the Universe and they try to understand the small difference between matter and antimatter, that might explain the origin of the matter in the Universe and why it contains no large amounts of antimatter. They also try to understand phase transitions that might have occurred in the early Universe, and might have analogies with cosmological inflation.

The proton synchrotron accelerator tunnel at CERN