La fisica degli ioni pesanti

When time began ...

... scientists believe there was a Big Bang from which everything in the Universe emerged. Fifteen thousand million years later, the Universe is so huge that it would take light millions of years cross. Yet in the beginning everything was squeezed into a tiny volume no bigger then a flea. All the particles which make up everyday matter, from which we and everything around us are made, had yet to form. The quarks and gluons, which in today's cold Universe are locked up inside protons and neutrons, would have been too hot to stick together. Matter in this state is called Quark Gluon Plasma, QGP, and experiments at CERN are trying to recreate it.

Scientists think that QGP might still exist today in the hearts of neutron stars which are so dense that a piece the size of a pinhead would weigh as much as a thousand jumbo jets. But even if QGP does exist there, it can't be studied, so to understand the first moments of the Universe's life scientists must create QGP in the laboratory. To do so, they smash ions, atoms stripped of electrons, into each other at very high energy, squeezing the protons and neutrons together to try and make them melt. Experiments at CERN through the 1980s and 1990s have smashed ions of oxygen, sulphur and lead into stationary targets. The results have given tantalizing hints that QGP might have been created for fleeting moments before cooling down into ordinary matter again.

NA45 detector

WA97 detector

The lead beam programme started in 1994, after the CERN accelerators has been upgraded by a collaboration between CERN and institutes in the Czech Republic, France, India, Italy, Germany, Sweden and Switzerland. A new lead ion source was linked to pre-existing, interconnected accelerators, at CERN, the Proton Synchrotron (PS) and the SPS. The seven large experiments involved measured different aspects of lead-lead and lead-gold collisions. They were named NA44, NA45, NA49, NA50, NA52, WA97/NA57 and WA98. Some of these experiments use multipurpose detectors to measure and correlate several of the more abundant observable phenomena. Others are dedicated experiments to detect rare signatures with high statistics. This co-ordinated effort using several complementing experiments has proven very successful.

NA50 detector

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.

At CERN's next accelerator, the LHC, lead ions will collide head-on at energies 300 times higher than at older CERN experiments. Physicists believe that at these energies making QGP will be a matter of routine allowing physicists to study its properties in detail.

What actually happens when a QGP forms? Let's take a look!