Heavy-ion physics

Abstract

Collisions of heavy ions at collider energies provide us with a unique opportunity to study strongly interacting matter at extreme temperatures and densities in the laboratory. Under these conditions quarks and gluons become deconfined to form a new state of matter, the quark-gluon plasma. Heavy ion physics has seen three major discoveries in the last 30 years. The first is that the QGP is the least dissipative material known and behaves like an almost perfect liquid. The second is that jets which are the manifestations of highly energetic quarks and gluons are strongly suppressed and modified compared to proton--proton collisions. This so-called jet quenching can be understood as the partial equilibration of a far-from-equilibrium system in a thermal QGP. The third main discovery is that particles with low transverse momentum produced in small collision systems like high multiplicity proton--proton and proton--ion collisions show many features that were believed to be signs for QGP formation. On the other hand, no jet quenching has been observed so far in small collision systems. These lectures are meant to give an overview over all relevant aspects of heavy ion physics at a phenomenological level.