Multifragmentation, Clustering, and Coalescence in Nuclear Collisions

Nuclear collisions at intermediate, relativistic, and ultra-relativistic energies offer unique opportunities to study in detail manifold fragmentation and clustering phenomena in dense nuclear matter. At intermediate energies, the well known processes of nuclear multifragmentation – the disintegration of bulk nuclear matter in clusters of a wide range of sizes and masses – allow the study of the critical point of the equation of state of nuclear matter. At very high energies, ultra-relativistic heavy-ion collisions offer a glimpse at the substructure of hadronic matter by crossing the phase boundary to the quark-gluon plasma. The hadronization of the quark-gluon plasma created in the fireball of a ultra-relativistic heavy-ion collision can be considered, again, as a clustering process. We will present two models which allow the simulation of nuclear multifragmentation and the hadronization via the formation of clusters in an interacting gas of quarks, and will discuss the importance of clustering to our understanding of hadronization in ultra-relativistic heavy-ion collisions. While most experimental studies concerning clustering and fragmentation of matter focus on the scale of atoms and molecules, there are prominent examples of these phenomena on the more fundamental scale of nuclear matter. In this note, we want to briefly present two of them: the multifragmentation transition for heated, diluted nuclear matter, and the clustering of quarks and hadrons at the transition from a quark-gluon-plasma to a gas of hadrons. The theoretical models we will use to study the relevant physics are the Quantum Molecular Dynamics (QMD) for nuclear matter, and the quark Molecular Dynamics (qMD) for the subnuclear degrees of freedom, respectively. We will further discuss how clustering helps to understand data from ultra-relativistic heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC), on the level of clustering both of partons and of hadrons.