When two lead ions collide at the Large Hadron Collider (LHC), they produce an extremely hot and dense state of matter in which quarks and gluons are not confined inside composite particles called hadrons. This fireball of particles—known as quark–gluon plasma and believed to have filled the universe in the first few millionths of a second after the Big Bang—expands and cools down rapidly. The quarks and gluons then transform back into hadrons, which fly out of the collision zone towards particle detectors.