Over fifteen years of painstaking detective work reached a watershed in 1999 as physicists from seven CERN experiments drew conclusions from the latest stage of the heavy-ion physics programme at the SPS. The heavy-ion programme was established with the aim of creating matter as it was just after the Big Bang. Back then, when the Universe was under 10 microseconds old, familiar particles like the protons and neutrons that make up everyday matter couldn't exist because it was simply too hot. The quarks that make up those familiar particles, along with the gluons that bind them together, are believed to have roamed freely in a kind of particle soup called Quark-Gluon Plasma (QGP).
In its bid to break down the forces that confine quarks to their particle cages and recreate QGP, CERN accelerated progressively heavier ions, starting with oxygen and moving up to lead, and smashed them into dense targets producing great concentrations of energy. Theory predicted a lot of different signals if QGP were made, so a range of experiments was designed to look for them all.
Lead ions, the heaviest ions ever accelerated, offered the best chance for SPS experiments to discover the QGP but first the accelerator complex had to be upgraded. Teams from CERN, the Czech Republic, France, Germany, India, Italy, Sweden and Switzerland set about the task and by 1994 a new lead ion source had been linked to CERN's accelerator complex. When the lead ions hit their targets, their collisions created temperatures over 100000 times higher than the centre of the Sun and energy densities twenty times that of ordinary nuclear matter.
Some were designed to look for correlations between several of the predicted QGP signals, others concentrated on specific signals. But all knew from the start that before any firm conclusion could be drawn, all the experiments would have to combine their results to put the whole picture together.
In 1999 the pieces were assembled and the combined result now gives strong evidence that a new state of matter has been created at CERN. This state of matter displays many of the features theory predicts for QGP, but nevertheless, this result is just the beginning. The CERN heavy-ion project has set new standards in international collaboration. It has brought together scientists from over twenty countries and fostered a productive partnership between high-energy physics and nuclear physics. The results from CERN lead naturally to future experiments designed to study this new state of matter in depth, clarifying our understanding of the early Universe. A heavy-ion programme at the Brookhaven laboratory in the USA is scheduled to start up in 2000, picking up where the SPS programme will soon leave off. Then in 2005, heavy-ion physics will return to CERN with the ALICE experiment at the LHC. To mark the transfer of the heavy-ion baton from CERN to Brookhaven, a special seminar is planned at CERN in 2000. Spokespeople of the seven SPS experiments will present their latest conclusions to the world, whilst wishing the best of success to Brookhaven.