Consolidated Grant

格拉斯哥核物理小组的研究方案侧重于研究强相互作用。作为自然界的四大基本力之一，强力负责原子核的形成和稳定。在更基本的层面上，它也是由夸克和胶子形成强子的相互作用，因此是宇宙中大部分可见质量的原因。量子色动力学(QCD)被广泛认为是描述强相互作用的基本理论；最近的诺贝尔奖(2004年，Gross，Politzer，Wilczek)因发展这一理论而获奖。QCD具有一些与电磁和弱相互作用理论非常不同的特征。只有非常高能的粒子物理过程才能很容易地被微扰计算，这一特征被称为渐近自由。在较低的能量下，可以应用包含QCD的一些基本对称性的有效场论，例如手征对称性。此外，还发展了夸克模型等模型，该模型将强相互作用粒子描述为三夸克或夸克-反夸克系统。在我们的研究中，我们使用散射实验来研究原子核和核子结构以及强子的光谱。我们在欧洲和美国的领先加速器设施进行实验：美国纽波特纽斯的Jefferson实验室；瑞典隆德的MAX-LAB；德国美因茨的Mami和德国汉堡的DESY。在这些实验中，我们使用(通常是极化的)电子束、正电子束和光子。我们的研究分为三个主题：-核子结构知道核子是由更基本的实体(夸克和胶子)组成的，我们需要建立它们内部的物质分布。形状因子和部分子分布函数用来描述核子结构。近年来，广义部分子分布(GPDs)的理论框架被发展起来，它系统地将核子结构的描述联系在一起。一旦测量，GPD将给我们一张核子的三维图像，以及一种获得核子内夸克的总角动量的方法。-强子能谱作为复合体，核子可以被激发到更高的质量态。虽然夸克模型描述了大量的激发光谱，但必须证实几个预测，以澄清夸克模型的哪一种变体最准确地描述现实。寻找预测的状态是一项非常困难的任务，除了其他技术外，还涉及使用偏振高能光子，类似于使用光学偏振来查看更多细节。对夸克模型以外的状态的观测对于回答为什么夸克和胶子从来没有被孤立地观察到这个问题具有根本的重要性，尽管有令人信服的证据表明它们肯定存在。这一特征被称为“限制”，是强相互作用所独有的，在任何其他自然基本力量中都没有观察到。这可以通过搜索所谓的胶球和奇异的混合介子来研究。-短程核结构我们想了解原子核、质子和中子(统称为核子)的成分是如何相互作用的，从而产生一系列现象。特别是，我们计划调查，当核子在核内碰撞中彼此非常接近时会发生什么，涉及3个核子的相互作用的强度，以及核介质如何影响在其中产生的粒子。我们还在研究少数体核，这可以用来检验手征有效场理论的预测。

The research programme of the Glasgow Nuclear Physics Group focuses on the study of the strong interaction. As one of the four fundamental forces in nature, the strong force is responsible for the formation and stability of atomic nuclei. At an even more fundamental level it also is the interaction that forms hadrons from quarks and gluons and is therefore responsible for most of the visible mass in the universe. Quantum Chromodynamics (QCD) is widely accepted as the fundamental theory describing the strong interaction; a recent Nobel Prize (2004, Gross, Politzer, Wilczek) was awarded for developing this theory. QCD has some features that make it very different from the theories of the electromagnetic and weak interactions. Only very high energy particle physics processes can easily be calculated pertubatively, a feature known as asymptotic freedom. At lower energies, effective field theories incorporating some of the fundamental symmetries of QCD, e.g. chiral symmetry, can be applied. In addition, models such as the quark model have been developed, which describes strongly interacting particles as either three-quark or quark-antiquark systems. In our research we use scattering experiments to investigate the structure of nuclei and nucleons as well as the spectrum of hadrons. We carry our experiments out at leading accelerator facilities in Europe and the US: Jefferson Lab in Newport News, USA; MAX-lab in Lund, Sweden; MAMI in Mainz, Germany and DESY in Hamburg, Germany. In these experiments we use (often polarised) beams of electrons, positrons and photons. Our research is organised into three themes: - Nucleon Structure Knowing that nucleons are made up of more fundamental entities (quarks and gluons), we need to establish the distribution of matter within them. Form factors and parton distribution functions are used to describe the structure of nucleons. In recent years the theoretical framework of Generalised Parton Distributions (GPDs) has been developed that ties the description of nucleon structure systematically together. Once measured, GPDs will give us a 3-dimensional picture of the nucleon as well as a way to access the total angular momentum of quarks inside a nucleon. - Hadron Spectroscopy As composite objects, nucleons can be excited to higher mass states. Whilst the quark model describes a great deal of the excitation spectrum, several predictions must be confirmed to clarify which variant of the quark model most accurately describes reality. Hunting for predicted states is a very difficult task, and involves, amongst other techniques, the use of polarised high energy photons similar to the way in which optical polarisation can be employed to see greater detail. The observation of states beyond the quark model is of fundamental importance in answering the question of why quarks and gluons have never been observed in isolation, even though there is compelling evidence that they must exist. This feature, known as 'confinement', is unique to the strong interaction, and is not observed in any of the other fundamental forces of nature. This can be studied by searching for so-called glueballs and exotic hybrid mesons. - Short-range Nuclear Structure We want to understand how the constituents of atomic nuclei, protons and neutrons (collectively known as nucleons), interact with each other to give rise to a wide range of phenomena. In particular we plan to investigate, what happens when nucleons pass very close to each other in collisions within a nucleus, the strength of interactions involving 3 nucleons and how the nuclear medium affects particles that are created within it. We are also studying few-body nuclei, which can be used to test predictions of Chiral effective field theories.