Jefferson Laboratory Upgrade Project

大约98%的核子质量，也就是可见宇宙的质量，都来自于它们的组分——夸克和胶子之间的相互作用。为裸夸克提供质量的希格斯机制只占核子质量的一小部分。夸克在介子和重子内的限制是它们基本相互作用的直接结果。强核力的场论，量子色动力学（QCD），现在已经很好地建立了，但是上面描述的现象还不能从量子色动力学的拉格朗日量来理解；它们是由这些相互作用的独特复杂性产生的自然属性。在核子和原子核的质量尺度上，QCD还很棘手，因此，在这些能量下，物质基本性质的清晰图景尚不存在。然而，这种情况将会改变，未来十年的理论和实验发展都有望在QCD和标准模型的背景下彻底改变我们对核物质的理解。这里提出的工作将解决几个关键问题：*在强相互作用粒子（强子）中限制夸克和胶子的机制是什么？质子和中子的结构是什么？强子的质量和自旋是如何产生的？我们能从夸克-夸克相互作用中理解强子的激发谱吗？*是否存在外来强子（多夸克态、混合介子和胶球）？* QCD如何产生核力？*能用我们对基本相互作用的理解来描述原子核吗？升级位于弗吉尼亚州纽波特纽斯的托马斯·杰斐逊国家加速器设施（JLab）背后的科学愿景是解决一些基本问题，比如组成夸克是如何获得质量的，以及它们为什么受到限制。因此，它非常适合处理上面列出的关键问题。对核子基本结构的新见解将从核子形状因子、广义部分子分布（gpd）和横向动量相关部分子分布（TMDs）的测量中获得。QCD约束的基本性质将通过对光强子谱的研究和寻找奇异态的存在来研究，在奇异态中，约束胶子场为夸克提供了额外的自由度。这些状态是QCD理论的直接预测，但在实验中尚未观察到。杰斐逊实验室12gev升级为英国核物理学界提供了一个令人兴奋的机会，可以领导和煽动世界领先的强子物理研究。开展这一项目的核心科学动机是对QCD物理学的持续渴望：获得关于核子结构的新信息，并研究夸克约束的机制。这一提议代表了爱丁堡和格拉斯哥核物理小组的一项出价，目的是在现有领导角色的基础上，增强他们在未来几十年对JLab未来科学项目的影响。

Approximately 98% of the mass of nucleons, and therefore of the visible universe, emerges from the interactions among their constituents, the quarks and gluons. The Higgs mechanism, which gives mass to the bare quarks, is responsible for only a small fraction of the nucleon mass. The confinement of quarks within mesons and baryons is a direct consequence of their fundamental interactions. The field theory of the strong nuclear force, Quantum Chromodynamics (QCD), is now well established, and yet the phenomena described above cannot be understood from the QCD Lagrangian; they are emergent properties that arise from the unique complexity of these interactions. QCD is as yet intractable at the mass scale of nucleons and nuclei, so a clear picture of the fundamental nature of matter at these energies does not yet exist. However, this situation is set to change, and both theoretical and experimental developments in the coming decade are expected to revolutionise our understanding of nuclear matter within the context of QCD and the Standard Model. There are several key questions that will be addressed by the work proposed here: * What is the mechanism for confining quarks and gluons in strongly interacting particles (hadrons)? * What is the structure of the proton and neutron and how do hadrons get their mass and spin? * Can we understand the excitation spectra of hadrons from the quark-quark interaction? * Do exotic hadrons (multiquark states, hybrid mesons and glueballs) exist? * How do nuclear forces arise from QCD?* Can nuclei be described in terms of our understanding of the underlying fundamental interactions?The scientific vision behind the upgrade to the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia, is to address fundamental issues such as how constituent quarks acquire mass, and why they are confined. It is therefore ideally suited to tackling the pivotal questions laid out above. New insights into the fundamental structure of the nucleon will be obtained from measurements of nucleon form factors, Generalised Parton Distributions (GPDs) and Transverse Momentum-dependent parton Distributions (TMDs). The fundamental nature of QCD confinement will be studied through the investigation of the light hadron spectrum, and the search for the existence of exotic states, in which the confining gluonic field provides an additional degree of freedom to the quarks. These states are a direct prediction of the QCD theory but remain unobserved experimentally. The Jefferson Lab 12 GeV Upgrade offers an exciting opportunity for the UK Nuclear Physics community to lead and instigate world-leading hadron physics research. The central scientific motivation for undertaking this project is a continuing desire to understand QCD physics: to obtain new information on the structure of nucleons, and to investigate the mechanism of quark confinement. This proposal represents a bid by the Edinburgh and Glasgow nuclear physics groups to build on established leadership roles, and enhance their impact on JLab's future science programme for the coming decades.