The primordial soup

High–energy collisions of lead ions

At a special seminar on 10 February 2000, spokespersons from the experiments on CERN's Heavy Ion programme presented compelling evidence for the existence of a new state of matter in which quarks, instead of being bound up into more complex particles such as protons and neutrons, are liberated to roam freely. Theory predicts that this primordial soup of quarks and gluons known as ‘quark-gluon plasma’ must have existed at about 10 microseconds after the Big Bang, before the formation of matter as we know it today.

The evidence for the new state of matter came from the combined data from 7 experiments in the heavy-ion programme at CERN. The aim was to smash very high-energy lead ions (33 TeV) from the Super Proton Synchrotron (SPS) into targets in the experiments, so as to create immensely high concentrations of energy and break down the forces that normally confine quarks inside more complex particles. The collisions created temperatures over 100 000 times as hot as the centre of the Sun, and energy densities 20 times that of ordinary nuclear matter - densities that have never before been reached in laboratory experiments. The data collected provided compelling evidence that a new state of matter had been created, featuring many of the characteristics of the theoretically predicted quark-gluon plasma.

The lead-beam programme started in 1994, with a new lead-ion source linked to the Proton Synchrotron (PS) and the SPS. Seven large experiments (NA44, NA45, NA49, NA50, NA52, WA97 / NA57 and WA98) measured different aspects of lead-lead and lead-gold collisions.

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.