Signatures of the QCD Phase Diagram

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The origin of matter and its properties in the present-day's universe is inextricably linked to the behavior of the strong interactions in an extremely hot or dense environment. We assume that the building blocks of nuclei and other so-called hadrons formed only microseconds after the Big Bang from the elementary particles of the strong interactions, the quarks and gluons. The early universe underwent a phase transition from the quark-gluon plasma (QGP) phase to the hadronic matter that surrounds us today. A main goal of the research on quantum chromodynamics (QCD) is to understand the underlying mechanism of this transition in order to establish a phase diagram of QCD analogous to the phase diagram of water. The decisive properties of QCD are confinement and chiral symmetry breaking. Confinement implies that quarks and gluons only exist in bound states, such as nucleons, in nature. Chiral symmetry breaking is responsible for 98% of the mass of the visible matter in the universe. It is commonly believed that these properties are not, or only partially, present in the QGP. On earth, this can only be investigated in ultra-relativistic heavy-ion collisions. Owing to the fact that quarks and gluons cannot be measured directly in such experiments, it is a formidable challenge to identify observables that carry signatures of the QCD phase transition to the particle detectors. Promising signatures of the QCD phase diagram, which are in the focus of this project, are particle number distributions and vector mesons. The former are particularly sensitive to the potential existence of an exceptional point in the phase diagram where hadrons and the QGP coexist. The latter have the potential to provide valuable informations about the mechanism of chiral symmetry breaking and thus about the origin of mass in the universe. This project aims at the development of quantitatively reliable theoretical description of the behavior of these signatures as they emerges from the underlying fundamental theory of the strong interactions. It focusses especially on the region of large density in the phase diagram. This is probed in current and future experiments, e.g., at the Large Hadron Collider at Cern, the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory or the Facility for Antiproton and Ion Research in Darmstadt, which is currently being built. However, due to the considerable technical difficulties that arise in the theoretical description of QCD at large densities, many things are still unknown in this region. The successful completion of this project will not only allow for the solid interpretation and prediction of experimental results also at large density, but also carries the potential to deepen our understanding of the underlying mechanisms of the formation of the matter around us.