Heavy-ion collisions are used at CERN and other laboratories to re-create conditions of high temperature and high energy density, similar to those that must have characterized the first instants of the universe, after the big bang. Yet heavy-ion collisions are not all equal. Because heavy ions are extended objects, the system created in a central head-on collision is different from that created in a peripheral collision, where the nuclei just graze each other. Measuring just how central such collisions are at the LHC is an important part of the studies by the ALICE experiment, which specializes in heavy-ion physics. The centrality determination provides a tool to compare ALICE measurements with those of other experiments and with theoretical calculations.
Read more: "Participants and spectators at the heavy-ion fireball" - CERN Courier