CERN Accelerating science

A few microseconds after the Big Bang, the universe was filled with an extremely hot and dense mixture of particles moving at near light speed. This matter was dominated by quarks and gluons that are subject to the strong force. Under usual circumstances, quarks are confined into hadrons. However, at extreme temperatures and densities, quarks and gluons can enter a deconfined state, forming the so-called quark-gluon plasma (QGP). To recreate the extreme conditions of matter which existed in the very early universe, we use powerful accelerators to make collisions between protons or heavy-ions, such as gold or lead nuclei. In the heavy-ion collisions hundreds of protons and neutrons in two such nuclei collide into each other to form a minuscule fireball in which everything melts into the quark-gluon plasma. The fireball then starts cooling and the individual quarks and gluons (collectively called partons) recombine into ordinary matter that flies away in all directions until they end up in our detectors. The final state of the collision contains many species of particles such as pions and kaons, which are mesons consisting of a quark-antiquark pair; or protons and neutrons, which are baryons containing three quarks; and even antiprotons and antineutrons, which may combine to form the nuclei of antiatoms as heavy as helium.