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绿色，作为卢卡斯教授斯蒂芬霍金的继任者。该小组投入了大量精力来开发在LHC中碰撞的质子内部夸克结构的精确模型，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签名事件，这项工作至关重要。我们将分析和解释LHC的结果，这些结果表明新的粒子，例如能量和动量被看不见的电中性粒子带走的事件。我们对寻找希格斯粒子特别感兴趣，它的质量似乎在LHC可以达到的范围内。希格斯粒子的一个显著信号是它频繁地衰变为两个底夸克（B），如果它有足够的质量，它会更频繁地衰变为两个顶夸克（t）（最重的类型）。重夸克物理学的其他方面也很有趣，我们正在使用超级计算机尽可能精确地计算含有B夸克的粒子的衰变率。对实验的谨慎可能揭示新的物理学。许多理论家希望看到的另一种新粒子是我们熟悉的粒子之一的“超对称”伙伴。超对称性是统一不同类型粒子的革命性思想。它不可能是自然界的精确对称，而且有许多模型可以说明超对称性可能只是近似真实的。我们已经开发了统计工具来区分它们，以应用于LHC以不断增加的能量产生数据。相信超对称性的原因是，它使我们的一些最强大的理论，量子场论和弦论，在数学上更加一致。场论提出了一些数学上的困难，如果在通常的三维空间之外还有额外的维度，情况会变得更糟。弦理论避免了这些困难中的一部分，只要它是超对称的，它在10维时空中是相容的。它有正确的结构来描述类似于我们所知道的粒子家族，并使引力理论与量子力学效应相一致，这是任何其他方式都无法实现的综合成功。我们仍在详细地检验弦理论是否能避免所有的困难，这些困难对场论来说虽然不是致命的，但却是麻烦的。我们还将探索从弦理论导出的非凡的规范/引力对应。这使我们能够研究物理世界中的现象，其中普通引力起着微不足道的作用，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，通过在非物理额外维度中进行引力计算。但这目前是有争议的，部分原因是证明它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，并寻找潜在的新应用。我们的一些工作涉及求解量子场论的经典近似方程，类似于求解光学中的麦克斯韦方程，而不是研究许多光子。这在数学上非常有趣。解决方案的例子是各种类型的涡旋，以及最近在奇异磁体中观察到的Skyrmions。Skyrme最初的想法是用Skyrmions来模拟原子核。它给出了一个介于刚性球形质子和中子团与无结构液滴之间的原子核的图像。我们将通过Skyrme图像对氦-4核（α粒子）之间的力以及激发核能级谱的预测来测试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.