Mapping Quantum Chromodynamics by Nuclear Collisions at High and Moderate Energies

The phase structure of strongly interacting matter in the temperature and density plane is expected to be very rich. At zero or small density/temperature ordinary nuclear matter is the so called confinement phase in which colored quarks and gluons, the building blocks of Quantum Chromodynamics (QCD), are confined in colorless hadrons. On the other hand, at high temperature and/or high density an exotic phase is expected in which color is deconfined and quarks can propagate in the medium on length scales larger than the typical hadron size. This new phase is named the quark-gluon plasma (QGP). The existence of this new phase, which should appear at a temperature of the order of 10^12 Kelvin, is proved theoretically by means of Lattice QCD as well as by means of effective models; the transition to this high energy phase can also be tested in current experiments by means of high energy nuclear collisions. ..Recent experimental data have suggested that a QGP-like medium might have been formed also in high energy proton-nucleus (pA) and proton-proton (pp) collisions. It is therefore of a certain interest to understand the dynamical evolution of the system created in these collisions from a theoretical point of view. One of the aim of this research project is to investigate on the early stage of high energy pp and pA collisions, with particular emphasis on the classical gluon field that are produced in the initial stage and on their evolution to an isotropic medium. After a clear understanding of the initial stage dynamics has been achieved, we aim to link our results to a relativistic transport evolution that will allow us to study the evolving system up to the hadronization time, and study how this initial dynamics affects experimental observables with particular emphasis on the two particle correlations and on hydrodynamic flows. ..At finite baryon chemical potential it is believed that the crossover to the high temperature phase is replaced by a first order phase transition: the point at which the two lines meet in the temperature/chemical potential plane is named the QCD critical point: therefore, it is very important to investigate on the possible measurable quantities to detect the critical point. Another part of the research will be devoted to the study of the QCD medium at finite baryon chemical potential. In particular, we will study the transport coefficients of the QCD medium at finite baryon chemical potential, then we will use the results to simulate the evolution of the medium created in low energy nuclear collisions. These collisions allow for the creation of a medium with a net baryon density and thus offer a way to map experimentally the QCD high chemical potential corner of the phase diagram. Moreover, they will potentially allow for the discovery of the QCD critical endpoint, namely the place in the phase diagram at which the crossover changes to a first order phase transition. Finally, they will allow for the experimental study of the QCD phases at finite baryon chemical potential, thus giving the possibility to test the effective models predictions about the possible phases of QCD at high density like the celebrated Quarkyonic phase in which quarks occupy large Fermi spheres but the thermal excitations on the top of these are colorless (baryon-like) excitations.

强相互作用在温度-密度平面上具有丰富的相结构。在低温或低密度时夸克和胶子被禁闭在强子中。而在高温或高密度时，人们预言一种退禁闭的相将会出现，这就是夸克胶子等离子体。根据格点QCD和有效模型的预言，这种相将会在温度达到10^12 K时出现。实验上也可以通过高能核碰撞来验证这一预言。最近的实验数据表明，夸克胶子等离子体物质可能在高能质子-原子核碰撞和质子-质子碰撞中产生。所以本项目中我们将着重关心碰撞后体系的动力学演化的理论解释。我们的研究目标之一是研究碰撞早期产生的胶子场以及由胶子场演化到各向同性物质的演化过程。在我们对碰撞早期动力学有了一个清楚的认识以后，我们将会把我们的研究成果和相对论输运演化理论相结合。这使我们可以研究从碰撞初期到强子化的全过程。也使我们可以研究初期动力学对实验可观测量的影响，我们将着重研究对双粒子关联和流体动力学流的影响。

强相互作用物质的在温度-密度平面上具有非常丰富的相结构。在零温/密度或者温度与密度都很小的情况下平常的核物质被称为禁闭相，此时具有色自由度的夸克与胶子（参与强相互作用的基本单元）被囚禁在对外不呈现色自由度的强子之中。另一方面，在高温或者高密度的情况下，人们预期存在一些奇特的相结构，色自由度不再呈禁闭状态，同时夸克可以在介质中传播，其运动尺度大于典型的强子大小，这种相叫做夸克胶子等离子体(QGP)。此种相结构应在10^12开尔文的左右的温度范围时出现，这已经被格点QCD方法以及多种有效模型在理论上证明。到如此高能的相变也被高能核碰撞实验证实。在零重子密度时，到QGP的相变实际上是一种平滑过渡，但很多研究指出在高化学势时，平滑过渡转变为一阶相变，因此应存在一个区分一阶相变与平滑过渡的临界点。在高能碰撞中，碰撞之后的相互作用区域立刻产生一系列强横胶子场，这被称为Glasma，作为此系统的初始条件。Glasma随着胶子场的动力学进行演化，我们建议，这种动力学可以通过重色探针进行探测，即重夸克：粲夸克和美夸克，其与Glasma同时形成，远早于夸克胶子等离子体。我们着眼于研究在核-核碰撞、质子-核碰撞以及质子-质子彭璜中的的重夸克在演化中的Glasma的传播问题。我们对重夸克扩散过程对可观测量（核修正因子以及椭圆流）的影响做出了预测。我们也通过一种新颖的方法，即热力学几何，研究了QCD相图。热力学几何方法应用以微分几何的语言表述的相变点附近的涨落理论，借此我们计算了QCD相变附近的热力学曲率，发现了曲率R蕴含着看看物质在临界点附近介观表现的重要信息。

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