Rolling Grant, Nuclear Physics Group, Glasgow Univ.

格拉斯哥核物理小组的研究项目侧重于强相互作用的研究。强力是自然界四大基本力之一，是原子核形成和稳定的关键。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观测质量的原因。量子色动力学（QCD）被广泛接受为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有一些特点，使它与电磁和弱相互作用的理论非常不同。只有非常高能量的粒子物理过程可以很容易地计算，这是一个被称为渐近自由的特征。在较低的能量下，有效的场论结合了QCD的一些基本对称性，例如手性对称性，可以应用。此外，像夸克模型这样的模型已经被开发出来，它将强相互作用的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的结构，以及核子共振和强子的光谱方法。这两种方法相辅相成。我们在欧洲和美国的领先加速器设施进行实验：瑞典隆德的max实验室；德国美因茨的MAMI；美国纽波特纽斯杰弗逊实验室；DESY在德国汉堡，FAIR在德国达姆施塔特。在这些实验中，我们使用（通常是极化的）电子束、光子束，以及（未来的）反质子束。我们的研究分为四个项目或主题：-短程核结构我们想要了解原子核，质子和中子（统称为核子）的成分如何相互作用，从而产生广泛的现象。特别是，我们计划研究，当核子在原子核内碰撞时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。-核子结构知道核子本身是由更基本的实体（夸克和胶子）组成的复合物体，我们需要建立它们内部物质的分布。形状因子和部分分布函数被用来描述核子的结构。近年来，人们建立了将核子结构描述系统地联系在一起的广义Parton分布理论框架。一旦测量，gpd将给我们一个核子的三维图像，以及一种获得核子内部夸克的总角动量的方法。作为复合物体，核子可以被激发到更高的质量状态。虽然夸克模型描述了大量的激发谱，但必须确认几个预测，以澄清哪种夸克模型最准确地描述了现实。寻找预测状态是一项非常困难的任务，在其他技术中，将涉及使用极化的高能光子，类似于光偏振可以用来看到更详细的信息。夸克模型之外的状态的观察对于回答为什么夸克和胶子从来没有被单独观察到的问题是至关重要的，尽管有令人信服的证据表明它们一定存在。这种被称为“约束”的特征是强相互作用所特有的，在自然界的任何其他基本力中都没有观察到。我们使用强子光谱的方法来寻找所谓的胶球和外来混合介子。

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 observable 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 spectroscopic methods for nucleon resonances and hadrons. Both approaches complement each other. We carry our experiments out at leading accelerator facilities in Europe and the US: MAX-lab in Lund, Sweden; MAMI in Mainz, Germany; Jefferson Lab in Newport News, USA; DESY in Hamburg, Germany and FAIR in Darmstadt, Germany. In these experiments we use (often polarised) beams of electrons, photons and also (in the future) anti-protons. Our research is organised into four programmes or themes: - 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. - Nucleon Structure Knowing that nucleons are themselves composite objects 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. - Nucleon Resonance 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 will involve, 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. - New Forms of Hadronic Matter 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. We use methods of hadron spectroscopy to search for so-called glueballs and exotic hybrid mesons.