基态八重态重子质量的手征描述

Lowest-lying octet baryon masses in covariant baryon chiral perturbation theory

质量作为物质最基本的属性,其起源一直是物理学研究的根本问题之一。根据物质层次的划分,理解可见宇宙物质(能量)组成的关键在于理解核子(基态重子)的质量。现代粒子物理与原子核物理研究指出,基态重子质量的绝大部分来自强相互作用。然而,由于色禁闭现象,强相互作用的基本理论–量子色动力学–在低能区无法解析求解,导致很长一段时间里对基态重子质量的研究不得不借助于各种唯象模型。二十一世纪以来,随着计算机运算能力的发展和算法的持续改进,格点量子色动力学模拟取得了令人瞩目的成绩,使得人类从第一原理出发计算基态重子的质量,进而定量地理解质量的起源成为可能。另一方面,受到计算资源的限制,目前绝大部分格点量子色动力学模拟必须采用比物理值大的轻夸克质量、不足够大的盒子体积和不足够小的格点间距。因而,为了从格点量子色动力学模拟中提取感兴趣的观测量的物理值,必须对格点数据进行如下三种延拓:手征延拓将轻夸克质量延拓到物理值(即mq→mphys.q),有限体积修正将有限的四维空间延拓到无穷大(即V=L4→∞),连续性延拓将有限格点间距延拓到零(即a→0)。手征微扰理论为开展这些延拓提供了必要的理论基础。作为量子色动力学的低能有效理论,手征微扰理论原则上可以模型无关地描述强相互作用物理。但随着手征阶数的升高,仅仅依靠实验数据无法完全确定理论中未知的低能常数。高统计量的格点量子色动力学模拟数据的出现为解决这一难题提供了新的思路,从而使得基于高阶手征微扰理论的研究成为可能。本文将简要介绍当前基于协变重子手征微扰理论对基态八重态重子质量及格点量子色动力学模拟数据的研究。

Mass is one of the most fundamental properties of matter. To understand its origin has long been a central topic in physics. The hierarchy in the composition of visible matter dictates that the key in understanding this issue is to understand the origin of the nucleon （lowest-lying baryon） masses. According to modern particle and nuclear physics, most of the ground state baryon masses come from the strong interaction between quarks and gluons. However, due to confinement, Quantum Chromodynamics（QCD）, the theory of the strong interaction, cannot be solved perturbatively. Therefore, for a long time, one has to utilize phenomenological models to understand the origin of baryon masses. In the past decade, with the increasing computing power and the continuous improvement of the numerical algorithms, lattice Quantum Chromodynamics simulations have been shown to be extremely successful in studying the nonperturbative regime of QCD. These simulations provide wonderful opportunities to understand the origin of mass from the first principles. On the other hand, due to （still） limited computing resources, most lattice calculations have to light quark masses larger than the physical ones, finite lattice volumes and lattice spacings. In order to extract the baryon masses at the physical point, one has to perform multiple extrapolations： chiral extrapolations, taking the quark masses to their physical values （mq→mq^php）, finite-volume corrections, connecting finite space-time with infinite space-time （V=L^4→∞）, and continuum extrapolations, sending the finite lattice spacing to zero （α→0）. Chiral perturbation theory, as the low-energy effective field theory of QCD, provides a model independent method to understand nonperturbative strong interaction physics and to guide the multiple extrapolations to connect the lattice QCD data to the physical world. At higher chiral orders, however, the relevant low-energy constants in chiral perturbation theory cannot be fully determined by studying only the experimental data. This issue can be solved with the help of high statistic lattice QCD data. In the present review, we will briefly introduce the latest systematic study of the lowest-lying octet baryon masses, particularly their lattice QCD simulations, in the covariant baryon chiral perturbation theory.