Heavy ion collision is the only controllable way currently to study the properties of strong interacting matters experimentally. By colliding two nuclei at sufficient high energy, it is possible to create a "fireball" with very high temperature and density that is actually a new state of matter: quark-gluon plasma(QGP). By studying the final particle production of the fireball, we can gain crucial knowledge about quantum chromodynamics, such as color confinement and chiral symmetry breaking.
The evolution of the "fireball" is very complicated, undergoing several phases with totally different characteristics, and the final observables are affected by many different factors. So theorists and experimenters work together to explain various phenomena. During the past decades, both experimental techniques and theoretical models have developed very rapidly. Yet new ideas and phenomena such as anomaly transport keep being brought forward, and this field is still full of new challenges and opportunities.