高温高密度时的新物质形态

New matter phases at high temperature and density

在高温高密条件下,夸克将解除禁闭形成全新的物质形态——夸克物质.本文从核物质的基本相互作用——量子色动力学(QCD)的基本对称性出发,首先简单介绍研究QCD相变的各种方法,阐述相变发生的物理机制和条件,然后着重讨论实现QCD相变的两种物理系统,即相对论重离子碰撞和致密星体.相对论重离子碰撞是制造高温环境的重要手段,核物质将形成全新的夸克胶子等离子体相(QGP).文中介绍了探测QGP的实验信号和高温环境特有的新奇物理现象.致密天体中的高密区则展现了极其丰富多彩的相结构.理论预示了与颜色和味道等自由度相联系的各类超导、超流态存在的可能性.相信在未来更大范围的束流能量扫描和对致密星体的精确观测中,我们将会对高温高密核物质的物态性质有着更加深入的认识.

It is widely accepted that a new phase structure will emerge in nuclear matter at high temperature and density. In this article the QCD （quantum chromodynamic） phase transitions are briefly reviewed from both theoretical and experimental aspects. Basing on the general principle of QCD and modern phase transition theory, we analyze potential phases in the strong interacting nuclear matter. The phase structure could be understood intuitively through the breaking and restora- tion of fundamental symmetries of QCD. In the space spanned by the temperature, baryon and isospin chemical potential there are different phases corresponding to three important symmetries, i.e. the color, chiral and isospin. At low tempera- ture and density, the color and isospin symmetries are preserved, while the chiral one is spontaneously broken. At high temperature or density, the color or isospin or both of them would be broken as the chiral restoration. In order to explore the detailed structure of the phase diagram various theoretical approaches of studying the transitions are introduced, such as lattice QCD, mean field calculation basing on effective models, functional renormalization group and Dy- son-Schwinger equation. In experiments there are two kinds of physics systems to realize conditions of QCD phase tran- sitions： the relativistic heavy ion collisions and compact stars. Nuclear collision is the only way to approach hot QCD in laboratories on the earth. We focus on some robust probes to the quark gluon plasma formed in the early stage of the col- lisions. There are generally two types of the traditional signals which are focusing on the profile of the fire ball and the internal interaction of it respectively. For the first case we introduce collective flows as observables for the shape of fire ball. While for the second we review the jet quenching and the heavy flavor suppression. In the hot fire ball the classical transport theory should be modified by including quantum effects induced by the chiral restoration. This novel transport phenomenon of QCD at high temperature is named by the chiral anomalous transport. On the phase transitions at high density, we review the recent progress on color superconductivity and pion superfluidity. Color superconductivity is formed at high baryon chemical potential when quarks pair with each other into diquarks and condense. While pion su- perfluidity would emerge at high isospin chemical potential with pion condensate. Model studies suggest these states are able to exist in the core of compact star. By modifying the structure of them, such as the mass-radius relation, these states are expected to be identified by astronomical observation at higher precision. Currently dense systems are difficult to produce at main facilities of the world, such as RHIC （relativistic heavy ion collider） and LHC （large hadron collider） because of the designed high energy scale of the accelerators. Therefore it is a critical need for the facilities which could cover the low and medium energy region where the potential critical point and various phase structures would locate. Considering the running and constructing heavy ion facilities, the CSR （cooler-storage-ring） in Lanzhou and the high intensity HIAF （heavy-ion accelerator facility） in Huizhou, the properties of the new QCD phases and their realizations in nuclear collisions will be the new frontier of nuclear physics in China.