Signatures of the QCD Phase Diagram

当今宇宙中物质的起源及其性质与极热或极密环境中强相互作用的行为密不可分。我们假设原子核和其他所谓强子的组成部分是在大爆炸后几微秒内由强相互作用的基本粒子——夸克和胶子——形成的。早期宇宙经历了从夸克-胶子等离子体（QGP）阶段到今天我们周围的强子物质的相变。量子色动力学（QCD）研究的一个主要目标是了解这种转变的潜在机制，以便建立类似于水的相图的QCD相图。QCD的决定性性质是约束和手性对称性破缺。约束意味着夸克和胶子在自然界中只存在于束缚态，比如核子。手性对称性破缺是宇宙中98%可见物质质量的原因。人们普遍认为，这些性质在QGP中不存在，或者只是部分存在。在地球上，这只能在超相对论重离子碰撞中进行研究。由于夸克和胶子不能在这样的实验中直接测量，因此确定携带QCD相变特征的可观测物到粒子探测器是一项艰巨的挑战。QCD相图的有希望的特征是粒子数分布和矢量介子，这是本项目的重点。前者对相图中强子和QGP共存的特殊点的潜在存在特别敏感。后者有可能提供有关手性对称性破缺机制的宝贵信息，从而了解宇宙中质量的起源。该项目旨在从强相互作用的基础理论中对这些特征的行为进行定量可靠的理论描述。它特别关注相图中密度较大的区域。这在当前和未来的实验中进行了探索，例如，在欧洲核子研究中心的大型强子对撞机，布鲁克海文国家实验室的相对论重离子对撞机或达姆施塔特的反质子和离子研究设施，目前正在建设中。然而，由于在大密度下QCD的理论描述中出现了相当大的技术困难，在这一领域仍有许多未知的事情。这个项目的成功完成不仅可以在大密度下对实验结果进行可靠的解释和预测，而且还具有加深我们对周围物质形成的潜在机制的理解的潜力。

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.