ALICE - A Large Ion Collider Experiment

When time began

Scientists believe there was a Big Bang from which everything in the Universe emerged. Fifteen billion years later, the Universe is so huge that it would take light billions of years to 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. Finding and studying it is ALICE's goal.

Pb+Pb heavy ion collision seen in NA49 detector

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, we can't reach it, so to understand the first moments of the Universe's life, scientists must create QGP in the laboratory. To do so, they colide 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.

At the LHC, lead ions will collide head-on at energies 300 times higher than at CERN's present day experiments. Physicists believe that these energies will be ideal for making QGP, allowing ALICE to study its properties in detail. A little bit of energy can knock atoms out of molecules or electrons out of atoms. With a little more energy, scientists can knock protons and neutrons out of atomic nuclei. But no matter how much energy they have, it appears to be impossible to knock an individual quark or gluon out of its proton or neutron cage. This confinement poses a problem for studying quarks and gluons. One approach is to increase the volume in which quarks and gluons are confined, so they behave as if they were free, or deconfined. By smashing lead ions together at high energy, this is what CERN aims to achieve in the ALICE experiment.

Deconfinement is a step on the way to making QGP, a mixture of quarks and gluons which has existed long enough for all the quarks and gluons to reach the same temperature. Think of what happens when you pour cold water into a hot bath. At first, there will be hot parts and cold parts, but with time, the temperature will even out. The bath will have thermalized. Similarly, it takes time for deconfined matter to thermalize.

The search for deconfined matter is young; it began in the 1980s with experiments coliding proton beams into proton or heavier targets. In today's experiments, beams of heavy-ions are used instead of protons. Each step to heavier particles and higher energies raises the energy density and temperature of the collision, increasing the chances of deconfinement.

ALICE in Cyberland

No one is absolutely sure what to expect when the transition from ordinary to deconfined matter occurs. Theorists predict different effects as matter heats up from its normal state into a deconfined one and cools down again. Over the years, CERN experiments have looked for all these effects. The results have been promising, but the temperatures currently achieved by smashing lead ions into lead targets appears to be only just enough to reach deconfinement. At the LHC lead ion collisions should heat matter up to temperatures at which QGP production becomes routine.

Heavy Ion Physics Challenge

The main challenge of heavy-ion physics is recording the enormous number of particles which emerge from the collisions. At CERN's present day energies, about 1500 particles are produced in each collision. At the LHC, this will go up to a staggering 50 000. A large fraction of these must be tracked and identified. Only then can a clear picture emerge, and key signals be found pointing to different stages in the evolution from ordinary matter to QGP and back again.

The Detector

The ALICE collaboration is currently building a state of the art detector optimized for heavy-ion physics.