Theoretical Particle Physics at the University of Liverpool

粒子物理学，也被称为高能物理学，研究在最短距离或最高能量下揭示的基本自然规律。近几十年来，无论是大规模的高精度实验，还是理论物理的研究，这一领域都取得了很大的进展。我们目前建立的知识被编码在所谓的标准模型中，这是一种基于规范不变性和希格斯机制(其中一个关键的结果，即希格斯粒子，尚未在实验中发现)的数学概念的高度非平凡的量子场论(QFT)。有很强的理由，包括宇宙学的暗示--这是一个现在与粒子物理密切相关的成熟研究领域--假设标准模型不是对自然的最终描述，即使在不久的将来可以获得的能源。此外，对于引力和粒子物理学中直接研究的其他基本力的一致描述，似乎需要一个以弦理论为主导的新的数学框架。随着欧洲核子研究中心大型强子对撞机的实验开始，高能物理现在进入了有望成为数十年来最令人兴奋的阶段。利物浦大学的理论物理小组目前由9名全职学术人员、3名全职研究人员(包括1名从行政和教学岗位退休的教授)和13名博士生组成，致力于与上述挑战相关的粒子物理的所有方面。我们的工作可以用三个研究小组来描述，主要涉及1.弦和超越标准模型(BSM)现象学和宇宙学，2.QFT中的高阶和全阶计算，以及对撞机上的QCD，3.低能强子物理，晶格QFT及其应用。由于弦理论在数学上的一致性，它要求空间的微观存在超过三个宏观维度。因此，第一组成员面临的一个主要挑战是理解这些额外维度的所谓紧凑，目的是获得LHC能量的预测并得出它们的宇宙学含义。弦理论中的另一个重要主题，也是我们小组所讨论的，是对黑洞的描述。现实弦理论需要一种新的高能对称性，称为超对称性(SUSY)，因此我们工作的另一个重点是研究标准模型的超对称性扩展。标准模型及其可能的扩展(例如SUSY)等QFT非常复杂，无法精确求解。对于像LHC这样的对撞机上的散射过程，唯一已知的方法是通过称为微扰理论的逐次近似，其中该理论的预测是根据一个小参数展开的。这种方法类似于简单数学函数的泰勒展开，但比它困难得多。第2组的成员在这些计算中发挥着国际领先的作用，这些计算不仅对于正确解释实验结果是不可或缺的，而且对于获得指导进一步研究的结构性见解也是不可或缺的。微扰理论不适用于强相互作用(QCD)理论中的许多量(例如静态)，在强相互作用理论中，基本粒子、夸克和胶子被“限制”在强子(如质子)中。格子理论在超级计算机上使用不连续的时空格子，是解决这些量的唯一的从头算方法。第三组的成员进行这样的计算和其他与禁闭和对可观测到的非微扰贡献有关的研究，特别是在这一点上为BSM物理提供了最有趣的线索之一的Muon的反常磁矩。

Particle physics, also called high-energy physics, addresses the fundamental laws of nature which are revealed at the shortest distances or highest energies. Great progress has been made in this field during recent decades, both by large-scale and high-precision experiments and by research in theoretical physics. Our presently established knowledge is codified in the so-called Standard Model, a highly non-trivial quantum field theory (QFT) based on the mathematical concepts of gauge invariance and the Higgs mechanism (of which a crucial consequence, the Higgs particle, has not yet been found in experiment). There are strong reasons, including hints from cosmology - a matured field of research now intimately connected to particle physics - to assume that the Standard Model is not the ultimate description of nature even at energies accessible in the near future. Furthermore a new mathematical framework, for which string theory is the leading candidate, appears to be required for a consistent description of gravity and the other fundamental forces directly studied in particle physics. With the start of experimentation at the LHC at CERN, a proton-proton collider with a multi-TeV centre-of mass energy, high-energy physics has now entered what promises to become its most exciting phase in decades. The Theoretical Physics Group at the University of Liverpool - at this point comprising 9 full-time academic staff members, 3 full-time researchers (including one professor retired from admin and teaching) and 13 PhD students - is engaged in all aspects of particle physics relevant to the challenges indicated above. Our work can be described in terms of three research groups, mainly addressing 1. String and Beyond the Standard-Model (BSM) phenomenology and cosmology, 2. Higher- and all-order calculations in QFT, and QCD at colliders, 3. Low-energy hadron physics, lattice QFT and applications. String theory requires, for its mathematical consistency, the microscopic existence of more than the three macroscopic dimensions of space. Therefore a main challenge, taken up by members of group 1, is to understand the so-called compactification of these extra dimensions, with the aims of obtaining predictions at LHC energies and deriving their cosmological implications. Another important topic in string theory, also addressed by our group, is the description of black holes. Realistic string theories require a new high-energy symmetry called supersymmetry (SUSY), so another focus of our work is the study of supersymmetric extensions of the Standard Model. QFTs such as the Standard Model and its possible extensions (e.g. SUSY) are so complicated that they cannot be solved exactly. For scattering processes at colliders such as the LHC, the only known method is by successive approximations called perturbation theory, where the predictions of the theory are expanded in terms of a small parameter. This method is analogous to, but much more difficult than, the Taylor expansion of simple mathematical functions. Members of group 2 play an internationally leading role in such calculations, which are not only indispensable for the correct interpretation of the experimental results, but also for gaining structural insights which will guide further research. Perturbation theory is not applicable to many (e.g. static) quantities in the theory of the strong interaction (QCD) where the fundamental particles, quarks and gluons, are `confined' in hadrons (bound states such as the proton). Lattice theory, which uses a discontinuous space-time lattice on supercomputers, is the only `ab initio' method to address such quantities. The members of group 3 perform such computations and other studies relevant to confinement and non-perturbative contributions to observables, in particular the anomalous magnetic moment of the muon which at this point offers one of the most intriguing hints for BSM physics.