Theoretical Particle Physics at the University of Liverpool

粒子物理学，也称为高能物理学，致力于在最短距离或最高能量下揭示的基本自然定律。近几十年来，无论是通过大规模、高精度的实验，还是通过理论物理的研究，这一领域都取得了很大的进展。我们目前已经建立的知识被编入所谓的标准模型，这是一种高度非平凡的量子场论（QFT），基于规范不变性和希格斯机制（希格斯机制的一个关键结果，希格斯粒子，尚未在实验中发现）的数学概念。有充分的理由，包括宇宙学的暗示--这是一个与粒子物理密切相关的成熟研究领域--假设标准模型不是自然的最终描述，即使在不久的将来可以获得的能量。此外，似乎还需要一个新的数学框架，弦理论是最好的候选者，来统一描述引力和粒子物理学中直接研究的其他基本力。随着欧洲核子研究中心（CERN）大型强子对撞机（LHC）实验的开始，高能物理学现在已经进入了几十年来最令人兴奋的阶段。在利物浦大学的理论物理组-在这一点上，包括9名全职学术人员，3名全职研究人员（包括一名教授从行政和教学退休）和13名博士生-是从事粒子物理学的所有方面相关的上述挑战。我们的工作可以描述为三个研究小组，主要解决1。弦和超越标准模型（BSM）现象学和宇宙学，2。QFT和QCD中的高阶和全阶计算，3。低能强子物理，晶格量子场论及其应用。弦论的数学一致性要求微观空间存在的维度多于宏观空间的三个维度。因此，第1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化，目的是获得LHC能量的预测并推导出它们的宇宙学含义。弦理论中的另一个重要课题，也是我们小组讨论的，是黑洞的描述。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扩展。QFT，如标准模型及其可能的扩展（如超对称性）是如此复杂，他们不能准确地解决。对于对撞机（如大型强子对撞机）的散射过程，唯一已知的方法是通过称为微扰理论的逐次逼近，其中理论的预测是以小参数的形式展开的。这种方法类似于简单数学函数的泰勒展开，但比泰勒展开困难得多。第2组的成员在这种计算中发挥着国际领先的作用，这不仅是正确解释实验结果所不可或缺的，而且也是获得结构见解，这将指导进一步的研究。微扰理论不适用于强相互作用理论（QCD）中的许多（例如静态）量，在强相互作用理论中，基本粒子，夸克和胶子，被“限制”在强子（束缚态，如质子）中。格点理论在超级计算机上使用不连续的时空格点，是解决这些量的唯一“从头算”方法。第3组的成员进行这样的计算和其他研究有关的约束和非微扰贡献的观测，特别是异常磁矩的μ子，在这一点上提供了一个最有趣的提示BSM物理。

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.