强磁场与涡旋场中的夸克胶子物质

相对论重离子碰撞可以产生高温的夸克胶子物质,同时也产生极强的电磁场和流体涡旋。在强电磁场和涡旋场中的夸克胶子物质呈现出新奇的宏观量子现象,比如手征磁效应、手征涡效应、手征分离效应、手征电分离效应、自旋极化现象等。它们一方面给我们提供了可以探测高温下量子色动力学的非平庸规范场拓扑结构、强相互作用的宇称破坏、夸克胶子物质中的自旋动力学等的实验手段,另一方面也与物理学其他分支,比如粒子物理、凝聚态物理、天体物理、冷原子物理等发生紧密联系,形成新的交叉研究领域。本文旨在对这些宏观量子现象的产生机制以及它们在相对论重离子碰撞中的探测等做一回顾和展望。特别地,我们揭示出重离子碰撞的磁场强度可以达到$10^{18}\sim 10^{20}$ G,流体涡旋可以达到$10^{22}$ s–1;这是我们已知当前宇宙中最强的磁场和流体涡旋。我们定量地对同量素碰撞实验做了分析,发现即便背景比例达到93%以上,当前的同量素碰撞实验仍然可在大约$3\sigma$的显著性水平上判断是否有手征磁效应的发生。我们系统地给出了满足因果律的自旋流体力学方程,并推导了其中的集体激发模式,这将有助于理解超子自旋极化中出现的符号问题。     

重离子碰撞中的矢量介子自旋排列

在非对心相对论重离子碰撞中,参与反应的系统具有巨大的轨道角动量,从而使产生的夸克胶子等离子体具有极强涡旋场,并通过自旋-轨道相互作用导致部分子的自旋极化,经过强子化导致重子的自旋极化以及矢量介子的自旋排列等可观测效应.矢量介子的自旋排列是指其自旋密度矩阵的00元素ρ00偏离1/3.在矢量介子衰变到两个赝标介子的过程中,衰变产物的极角分布只与ρ00有关,以此可以对自旋排列进行测量.理论研究表明,重离子碰撞过程中,重子的自旋极化反映了夸克自旋极化的时空平均效应,而矢量介子自旋排列则反映了夸克反夸克自旋极化的局域相空间关联.本文回顾了相对论重离子碰撞中矢量介子自旋排列的相关理论工作.重点以非相对论夸克融合模型为例,明确地计入夸克极化的相空间依赖性,展示了矢量介子自旋排列与夸克反夸克自旋极化特别是它们之间相空间关联的关系.本文还讨论了涡旋、电磁场、有效φ介子场以及它们的局域涨落对φ介子自旋排列的贡献,结果显示强作用场的时空关联效应是导致φ介子自旋排列的主要因素.矢量介子自旋排列为探索强相互作用物质和强相互作用场的性质提供了新途径.     

强相互作用自旋-轨道耦合与夸克-胶子等离子体整体极化

在非对心相对论重离子碰撞中,参与反应的原子核物质系统具有巨大的初始轨道角动量,经过强相互作用的自旋-轨道耦合,这一巨大的轨道角动量可以转化为产生的夸克-胶子等离子体的整体极化.整体极化效应在理论上提出后,首先被美国布鲁克海文国家实验室的相对论重离子对撞机上的STAR实验所证实,激发了人们对相关问题的研究,成为重离子碰撞物理研究的一个新方向——重离子碰撞自旋物理.本文简单回顾了整体极化原始基本思想、理论计算体系与主要结果以及近几年的理论进展.     

中高能重离子碰撞中的电磁场效应和手征反常现象

重离子碰撞可以产生极强的电磁场和高温高密量子色动力学(QCD)物质,诱导很多重要手征反常现象,例如手征磁效应和手征磁波.本文围绕手征反常现象中的诸多物理要素,详细介绍包括相对论重离子碰撞中不同碰撞系统和能量下的电磁场特性、同质异位素碰撞中寻找手征磁效应、手征磁波特性、中低能重离子碰撞中磁场效应等一系列与电磁场和手征反常现象相关的理论研究成果.相关研究有助于实验中寻找强相互作用中的电荷宇称(CP)破缺的证据,加深对QCD真空涨落和宇宙中正反物质不对称问题的理解.     

(S_(NN))~(1/2)=130GeV时Au Au碰撞中的反重子与重子比率

按照夸克随机组合的简单组合模型,本文通过分析得出重子反重子比率与夸克反夸克平均产生几率的关系．进一步计算了相对论重离子碰撞过程中各种反粒子与粒子的比率,不需要任何额外假定, 预言的比率与RHIC加速器时Au Au碰撞STAR实验的结果基本一致．表明夸克组合模型能够解释相对论重离子碰撞过程的反粒子与粒子比率．     

相对论自旋流体力学

近年来,随着重离子碰撞实验中超子自旋极化与矢量介子自旋排列现象的发现,关于夸克胶子物质中自旋输运的理论研究也得到蓬勃发展,其中包括相对论自旋流体力学,它是描述自旋输运的流体力学理论.本文对相对论自旋流体力学的近期发展进行了综述,主要包括以下内容:1)相对论自旋流体力学基本方程的推导,包括宏观的唯象学推导、基于有效场论的推导以及基于输运理论的推导; 2)该理论框架的一些特殊性质,包括能动量张量中的反对称结构以及赝规范变换性质等; 3)在Bjorken和Gubser膨胀体系中的解析解及其对于重离子碰撞物理的意义.     

√SNN=130GeV时Au+Au碰撞中的反重子与重子比率

按照夸克随机组合的简单组合模型, 本文通过分析得出重子反重子比率与夸克反夸克平均产生几率的关系. 进一步计算了相对论重离子碰撞过程中各种反粒子与粒子的比率, 不需要任何额外假定, 预言的比率与RHIC加速器√S_NN=130GeV时Au+Au碰撞STAR实验的结果基本一致. 表明夸克组合模型能够解释相对论重离子碰撞过程的反粒子与粒子比率.     

RHIC能区Au+Au碰撞中带电粒子直接流与超子整体极化的计算与分析

非对心的相对论重离子碰撞中,不参与碰撞的核子会对参与碰撞的核子产生纵向拖拽,形成一个相对于纵向倾斜的夸克胶子等离子体(QGP)火球.同时,对撞的原子核可将巨大的轨道角动量沉积于QGP中,使其中的部分子沿系统总角动量方向发生自旋极化.在光学Glauber模型基础上,本文构建了倾斜的三维QGP初态条件,并结合3+1维黏滞流体力学模型CLVisc,研究了重离子碰撞的末态带电粒子的直接流和Λ/■超子的整体极化.计算表明,倾斜的初态条件与流体力学模型的结合能够较好地描述RHIC-STAR实验上观测到的直接流与超子整体自旋极化的数据.这为人们利用这些观测量进一步约束重离子碰撞产生的核物质的初始几何与运动学状态提供了理论依据.     

费米子的相对论自旋输运理论

在重离子碰撞中,自旋轨道耦合可以导致整体极化现象.自从2017年,STAR工作中发现超子Λ在Au+Au碰撞中的整体极化,整体极化效应引起了学术界的广泛关注.整体极化效应的微观产生机制可以利用粒子之间非定域的散射过程来描述：在重离子碰撞中产生了热密物质,热密物质中的粒子之间通过非定域的碰撞过程实现了轨道角动量向自旋角动量的转换,从而导致散射后的粒子自旋极化.为了描述这一微观过程,在相空间描述自旋轨道耦合更加方便,而自旋轨道耦合又是一种量子效应,所以基于协变维格纳函数的量子动理学理论将是描述整体极化现象的有力工具.本文介绍了基于维格纳函数的量子动理学理论以及自旋输运理论.近期自旋输运理论的发展为以后数值模拟自旋极化现象的时空演化提供了理论基础.     

量子色动力学在有限温度密度区间的相结构

本文综述量子色动力学在有限温度密度区间的相结构,包括高密条件下的色超导态和在高温条件下相对论重离子碰撞实验中产生的强耦合夸克-胶子等离子体。我们简要地介绍在相对论重离子碰撞实验中发现强耦合夸克-胶子等离子体的历史,并且讨论用体粘滞系数与熵的比值确定QCD临界点的可能性。对于在高密区域的相结构,我们着重讨论错位配对情况下的非标准色超导态,解释无能隙色超导态的色磁不稳定性、Sarma不稳定性和Higgs不稳定性.     

相对论重离子碰撞的输运模型研究

美国的布鲁克海文国家实验室相对论重离子对撞机（RHIC）和欧洲核子中心的大型强子对撞机（LHC）的大量实验结果表明,在相对论重离子碰撞中已经产生了一种近似完美流体的强耦合部分子物质。基于一个多相粒子输运模型（AMPT）理论工具,对RHIC和LHC实验上的一些重要结果的开展了三个方面的理论研究工作（集体流、喷注淬火、手征磁效应）,研究结果揭示了初始的夸克胶子等离子体（QGP）能量密度涨落经过部分子输运演化产生末态粒子的各阶次的集体流、喷注和部分子物质的相互作用导致喷注的能量损失、末态相互作用严重影响手征磁效应的大小等物理过程作用机制。The experimental results from the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) show that a nearly perfect fluid (i.e. strong-coupling Quark Gluon Plasma) has been created in relativistic heavy-ion collisions. I introduce our theoretical results based on a multi-phase transport (AMPT) model. Several important topics such as collective flow, jet quenching, chiral magnetic effect, are addressed. The simulation results indicate that the initial fluctuations of energy density of the QGP lead to all orders of harmonic flows of final particles via parton cascade, the strong interactions between jet and the QGP make jet lose much energy, and the final state interactions play an important role to affect the initial chiral magnetic effect in relativistic heavy-ion collisions.     

Estimation of the chiral magnetic effect considering the magnetic field response of the QGP medium

The magnetic field plays a major role in searching for the chiral magnetic effect in relativistic heavy-ion collisions. If the lifetime of the magnetic field is too short, as predicted by simulations of the field in vacuum, the chiral magnetic effect will be largely suppressed. However, the lifetime of the magnetic field will become longer when the QGP medium response is considered. We give an estimate of the effect, especially considering the magnetic field response of the QGP medium, and compare it with the experimental results for the background-subtracted correlator H at RHIC and LHC energies. The results show that our method explains the experimental results better at the top RHIC energy than at the LHC energy.     

Enhancement of high PT hadrons due to collapsing Z(3) walls in heavy-ion collisions

We discuss enhancement of multiplicities of hadrons at high transverse momentum due to multiple reflections of quarks from collapsing Z(3) interfaces in the QGP produced in relativistic heavy-ion collisions. By modeling the dependence of effective mass of the quarks on the Polyakov loop order parameter, we evaluate the reflection coefficient of quarks from collapsing Z(3) interfaces. We use the effective potential proposed by Pisarski for the Polyakov loop to determine the profile of the Z(3) interfaces and calculate the reflection probability for quarks. We discuss the formation of a network of these Z(3) walls in relativistic heavy-ion collisions, in the QGP phase. We do a numerical simulation to calculate the modifications in the thermal transverse momentum spectra of the quarks/anti-quarks that results from a collapsing wall. We then use the recombination model to calculate the transverse momentum spectrum of final hadrons. Our results show enhancement of high P _T hadrons, with the enhancement being stronger for heavier quarks. Further, we find that due to larger reflection coefficient for heavier quarks, the density of strange and charm quarks/anti-quarks increases inside the collapsing walls. This implies enhancement in the multiplicities of multi-strange and multi-charmed hadrons.     

Azimuthal anisotropy and formation of an extreme state of strongly interacting matter at the relativistic heavy-ion collider (RHIC)

Experimental results obtained by studying the azimuthal anisotropy of final states in nucleus-nucleus interactions at the energies of the relativistic heavy-ion collider (RHIC) are systematized. The medium is found to exhibit a pronounced collective behavior, which is likely to be formed at an early, parton, stage of the spacetime evolution of product hot and dense matter. Experimental data on the azimuthal anisotropy indicate that strongly interacting matter produced in the final state under extreme conditions behaves as a nearly ideal liquid rather than an ideal gas of quarks and gluons. The experimentally observed suppression of high-transverse-momentum jets and substantial modification of jetlike azimuthal correlations in heavy-ion collisions suggest that the energy loss of partons propagating in high-temperature matter featuring a high density of color charges is extremely large. The dependence of the amount of hardjet suppression in nucleus-nucleus collisions on the orientation of a jet with respect to the reaction plane was first discovered experimentally at RHIC. A strong suppression of the production of high-transverse-momentum particles and jets at RHIC is a unique phenomenon, which was discovered experimentally at lower energies.     

Polarization of Λ0 hyperons as a signature for the quark-gluon plasma

The momentum distribution of Λ⁰ hyperons produced from the quark-gluon plasma (QGP) in ultra-relativistic heavy-ion collisions is calculated in dependence on their polarization. The momentum distribution of Λ⁰ hyperons is defined by matrix elements of relativistic quark Wigner operators, which are calculated within the effective quark model with chiral U(3)×U(3) symmetry and the quark-gluon transport theory. We show that the polarization of the Λ⁰ hyperon depends on the spin of the strange quark that agrees well with the DeGrand-Miettinen model. We show that Λ⁰ hyperons, produced from the QGP, are fully unpolarized. This means that a detection of unpolarized Λ⁰ hyperons, produced in ultra-relativistic heavy-ion collisions, should serve as one of the signatures for the existence of the QGP in intermediate states of ultra-relativistic heavy-ion collisions.     

Quark-hadron three-phase mixture of baryonic matter

The phase diagrams of strongly interacting baryonic matter are constructed in a phenomenological model assuming three phases: hadrons, quarkgluon plasma and constituent quarks. Three-phase mixture states are considered in detail and it is shown that these states cover a finite area in the energy density-baryonic density plane. Possible hydrodynamic expansion processes involving the three-phase coexistence region in relativistic heavy-ion collisions are outlined.     

Dynamical effects of spin-dependent interactions in low- and intermediate-energy heavy-ion reactions

It is well known that noncentral nuclear forces, such as the spin–orbital coupling and the tensor force, play important roles in understanding many interesting features of nuclear structures. However, their dynamical effects in nuclear reactions are poorly known because only the spin-averaged observables are normally studied both experimentally and theoretically. Realizing that spin-sensitive observables in nuclear reactions may convey useful information about the in-medium properties of noncentral nuclear interactions, besides earlier studies using the time-dependent Hartree–Fock approach to understand the effects of spin–orbital coupling on the threshold energy and spin polarization in fusion reactions, some efforts have been made recently to explore the dynamical effects of noncentral nuclear forces in intermediate-energy heavy-ion collisions using transport models. The focus of these studies has been on investigating signatures of the density and isospin dependence of the form factor in the spin-dependent single-nucleon potential. Interestingly, some useful probes were identified in the model studies but so far there are still no data to compare with. In this brief review, we summarize the main physics motivations as well as the recent progress in understanding the spin dynamics and identifying spin-sensitive observables in heavy-ion reactions at intermediate energies. We hope the interesting, important, and new physics potentials identified in the spin dynamics of heavy-ion collisions will stimulate more experimental work in this direction.