1986: heavy–ion collisions begin

Theory predicts that just after the Big Bang the Universe was too hot and dense for the existence of familiar particles such as protons and neutrons. Instead, their constituents – the quarks and gluons – roamed freely in a ‘particle soup’ called quark-gluon (QGP) plasma.

To investigate this possibility, in 1986 CERN began to accelerate heavy ions – nuclei containing many neutrons and protons – in the Super Proton Synchrotron (SPS). The aim was to ‘deconfine’ the quarks by smashing the heavy ions into appropriate targets. The first experiments used relatively light nuclei such as oxygen and sulphur, and produced results consistent with the QGP interpretation, but no real proof. In 1994 a second generation of experiments began with lead ions, and by 2000 there was compelling evidence that a new state of matter had indeed been seen.

The last heavy–ion run at the SPS took place in 2003. The next big step at CERN will be with lead-ion collisions in the Large Hadron Collider and the ALICE experiment in particular.