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Kinetics of QCD phase transitions in effective models

有效模型中 QCD 相变动力学

基本信息

批准号：
250903567
负责人：
Professor Dr. Carsten Greiner
金额：
--
依托单位：
Institut für Theoretische Physik
依托单位国家：
德国
项目类别：
Research Grants
财政年份：
2014
资助国家：
德国
起止时间：
2013-12-31 至 2017-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/250903567?language=en
关键词：
Kinetics QCD phase transitions effective

项目摘要

Within the standard model of elementary particles the strong interaction is described by quantum chromodynamics (QCD), which is a non-Abelian gauge theory showing the fundamental properties of asymptotic freedom (i.e., the effective coupling becomes weak at collisions with large energy-momentum transfer) and confinement (i.e., the elementary constituents, the quark and gluons, cannot be observed as free particles but are bound in hadrons (mesons as bound states of a quark and an anti-quark and baryons as bound states of three quarks) that carry vanishing net-color charge.Thus, one expects that in a hot and dense medium of strongly interacting particles at some critical temperature and/or density the quarks and gluons can move quasi freely within that medium. This implies that a kind of phase transition occurs where the relevant degrees of freedom are not anymore hadrons but quarks and gluons ("partons"). This state of matter is called the quark-gluon plasma (QGP).In addition to the fundamental local color-gauge symmetry, in the light-quark sector (up, down, strange quarks) QCD is also characterized by an approximate chiral symmetry, which is spontaneously broken in the vacuum and at low temperatures and densities due to the formation of a quark condensate. At high temperatures one expects a "melting" of this quark condensate and thus the restoration of chiral symmetry.From lattice-QCD calculations in thermal equilibrium on knows that at high temperatures and low net-baryon densities the phase transitions are continuous "crossover transitions", while effective hadronic models suggest that at high temperatures and/or net-baryon densities the phase transition becomes of 1st order. This suggests further that this line of a first-order phase transition ends in a critical point, where the phase transition becomes of 2nd order.Now one hopes to find signatures of such first-order phase transitions or even a critical point in ultrarelativistic heavy-ion collisions. Possible signatures are fluctuations of conserved quantum numbers like the net-baryon number which are quantified by comulants of the corresponding statistical distributions. The main goal of our project is the quantum-field theoretical investigation whether such fluctuations can build up in a finite system of finite lifetime as the "fireballs" created in heavy-ion collisions. To that end we want to investigate effective quantum-field theoretical models (like the chiral quark-meson model) in nonequilibrium situations close to phase transitions and at the critical point, using the real-time formulation of relativistic many-body quantum-field theory (Schwinger-Keldysh formalism) both analytically (by deriving the appropriate Kadanoff-Baym equations and related semi-classical transport equations) and in computer simulations.

在基本粒子的标准模型中，强相互作用由量子色动力学（QCD）描述，这是一种非阿贝尔规范理论，显示了渐近自由的基本性质（即，有效耦合在具有大的能量-动量转移的碰撞时变弱）和约束（即，夸克和胶子这两种基本成分不能作为自由粒子被观察到，它们被束缚在强子中（介子作为一个夸克和一个反夸克的束缚态，重子作为三个夸克的束缚态），它们携带消失的净色电荷。因此，人们预期，在某一临界温度和/或温度下，在由强相互作用粒子组成的热的稠密介质中，夸克和胶子可以在这种介质中准自由地运动。这意味着发生了一种相变，其中相关的自由度不再是强子，而是夸克和胶子（“部分子”）。这种物质状态被称为夸克胶子等离子体（QGP）。除了基本的局域色规范对称性之外，在轻夸克扇区（上夸克、下夸克、奇夸克）中，QCD还具有近似手征对称性的特征，这种对称性在真空、低温和低密度下由于夸克凝聚的形成而自发破缺。在高温下，这种夸克凝聚体会发生“熔化”，从而恢复手征对称性。从热平衡时的晶格QCD计算可知，在高温和低的净重子密度下，相变是连续的“交叉跃迁”，而有效的强子模型表明，在高温和/或净重子密度下，相变变为一阶相变。这进一步表明，这条一级相变线终止于临界点，在临界点处相变变为二级相变。现在人们希望在超相对论重离子碰撞中找到这种一级相变的特征，甚至是临界点。可能的签名是保守量子数的波动，如净重子数，由相应的统计分布的余量量化。我们的项目的主要目标是量子场理论研究是否可以建立在一个有限的系统中的有限寿命的重离子碰撞中产生的“火球”这样的波动。为此，我们希望研究有效的量子场论模型（如手征夸克-介子模型）在非平衡情况下接近相变和在临界点，使用实时制定的相对论多体量子场论（Schwinger-Keldysh形式主义）分析（通过导出适当的Kadanoff-Baym方程和相关的半经典输运方程）和计算机模拟。