Investigations in Particle Physics Theory

剑桥粒子物理理论小组有着广泛的兴趣，包括在欧洲核子研究中心的大型强子对撞机(LHC)上寻找新的粒子，理解弦理论，以及研究量子场论中出现的不同类型的粒子。我们的成员之一是弦理论家迈克尔·格林，他接替斯蒂芬·霍金担任卢卡斯教授。该小组致力于开发大型强子对撞质子内部夸克结构的准确模型，并预测质子-质子碰撞的结果。如果要在实验中识别涉及新粒子的标志性事件，这项工作是至关重要的。我们将分析和解释LHC的结果，这些结果表明有新的粒子，例如，能量和动量被看不见的电中性粒子带走的事件。我们特别感兴趣的是寻找希格斯粒子，它的质量似乎在大型强子对撞机可以到达的范围内。希格斯粒子的一个引人注目的信号是它频繁衰变到两个底部(B)夸克，如果它有足够的质量，它甚至会更频繁地衰变成两个顶部(T)夸克(最重的类型)。重夸克物理的其他方面也很有趣，我们正在使用超级计算机尽可能准确地计算包含b夸克的粒子的衰减率。与实验的矛盾可能会揭示新的物理学。许多理论家期待看到的另一种新粒子是我们所熟悉的其中一种粒子的“超对称”伙伴。超对称性是统一不同类型粒子的革命性想法。它不可能是自然界的精确对称性，有许多模型可以说明超对称性如何可能只是近似地正确。我们已经开发了统计工具来区分它们，以便在大型强子对撞机以不断增加的能量产生数据时应用。相信超对称性的原因是，它使我们一些最强大的理论，包括量子场论和弦理论，在数学上更加一致。场论带来了一些数学上的困难，如果有超出通常的三个维度的额外空间维度，情况会变得更糟。弦理论避免了其中的一些困难，并且在十个时空维度上是一致的，前提是它是超对称的。它具有合适的结构来描述与我们所知的粒子族相似的粒子族，并使引力理论与量子力学效应保持一致，这是任何其他方式都无法实现的共同成功。我们仍在详细测试弦理论是否避免了场论的所有困难，尽管这些困难对场论来说是麻烦的，但不是致命的。我们还将探索从弦理论派生出来的显著的规范/重力对应。这使我们能够通过在非物理额外维度进行重力计算，来研究物理世界中普通引力发挥着微不足道作用的现象，如夸克-胶子等离子体状态、原子核的结构，以及奇异超导体中的某些效应。但这目前是有争议的，部分原因是证明其合理性的严格结果依赖于精确的超对称性。我们将努力了解它的应用范围有多广，并寻找潜在的新应用。我们的一些工作涉及在经典近似的量子场论中求解方程，类似于求解光学中的麦克斯韦方程，而不是研究许多光子。这在数学上是非常有趣的。解决方案的例子有各种类型的涡旋，以及最近在奇异磁铁中观察到的天空微子。Skyrme最初的想法是用Skyrmions来模拟原子核。它给出了一张介于一团坚硬的球形质子和中子和一个无结构液滴之间的原子核的图片。我们将通过对氦-4核(阿尔法粒子)之间的作用力和激发核的能级光谱的预测来测试Skyrme图像。

The Cambridge Particle Physics Theory group has broad interests including searching for new particles at the Large Hadron Collider (LHC) at CERN, understanding string theory, and studying the different types of particles that occur in quantum field theories. One of our members is string theorist Michael Green, successor to Stephen Hawking as Lucasian Professor. The group has devoted much effort to developing accurate models of the quark structure inside the protons that collide at LHC, and predicting the outcome of the proton-proton collisions. This work is vital if the signature events involving new particles are to be recognised in the experiments. We will analyse and interpret results from LHC that suggest new particles, for example events where energy and momentum is carried away by unseen, electrically neutral particles. We are particularly interested in searches for the Higgs particle, whose mass appears to be in the range that LHC can reach. A striking signal of a Higgs particle is its frequent decay to two bottom (b) quarks, and it would decay even more frequently into two top (t) quarks (the heaviest type) if it had enough mass. Other aspects of heavy quark physics are also interesting and we are using supercomputers to calculate as accurately as possible the decay rates of particles that contain b quarks. Discrepancy with experiment could reveal new physics. Another kind of new particle that many theorists expect to see is a 'supersymmetric' partner of one of the particles familiar to us. Supersymmetry is a revolutionary idea for unifying particles of different types. It cannot be an exact symmetry of nature and there are many models for how supersymmetry may be only approximately true. We have developed statistical tools to distinguish among them, to be applied as LHC produces data at ever increasing energies. The reason for believing in supersymmetry is that it makes some of our most powerful theories, both quantum field theories and string theory, mathematically more consistent. Field theories present some mathematical difficulties, which get worse if there are extra dimensions of space beyond the usual three. String theory avoids some of these difficulties, and is consistent in ten spacetime dimensions, provided it is supersymmetric. It has the right structure to describe families of particles similar to those we know, and to make gravity theory consistent with quantum mechanical effects, a combined success not achieved in any other way. We are still testing in detail whether string theory avoids all the difficulties which are troublesome, though not fatal, for field theories. We will also explore the remarkable gauge/gravity correspondence, derived from string theory. This allows us to study phenomena in the physical world in which ordinary gravity plays a negligible role, like the quark-gluon plasma state, the structure of atomic nuclei, and certain effects in exotic superconductors, by carrying out gravity calculations in unphysical extra dimensions. But this is currently controversial, partly because the rigorous results justifying it rely on exact supersymmetry. We will be trying to understand how widely it can be applied, as well as looking for potential new applications. Some of our work involves solving equations in the classical approximation to quantum field theory, analogous to solving Maxwell's equations in optics rather than studying many photons. This is mathematically very interesting. Examples of solutions are various types of vortices, and the Skyrmions that were observed recently in exotic magnets. Skyrme's original idea was to model nuclei by Skyrmions. It gives a picture of a nucleus intermediate between a cluster of rigid, spherical protons and neutrons, and a structureless liquid drop. We will test the Skyrme picture through its predictions for the force between Helium-4 nuclei (alpha particles), and for the spectra of energy levels of excited nuclei.