Investigations in Theoretical Particle Physics

大型强子对撞机（LHC）将于今年开始运行，以比以往更大的能量碰撞质子。通过爱因斯坦著名的方程E = mc^2 $，这种更大的能量可以转化为比以往任何时候都更重的粒子质量。十年前的实验暗示，至少希格斯粒子，或者类似的东西，应该在大型强子对撞机上产生。剑桥小组正在开发技术，从产生的数十亿其他粒子中提取出希格斯粒子的几个签名衰变。我们将研究如何从数据中提取希格斯粒子的性质，包括它的自旋，理论家预测它的自旋为零。建立精确的计算机模拟碰撞和随后的衰变是一项重要的活动。在我们目前的理论中，希格斯粒子是所有其他基本粒子质量的起源，但为什么希格斯粒子的质量并不比它重得多，这是一个谜。诸如超对称性或额外空间维度之类的推测性想法可以解决这个难题，但我们不能相信这些，除非它们的影响得到明确的测量。一个可能的效应是在LHC中产生暗物质粒子，其特征是粒子碰撞中的“丢失能量”。我们需要调查任何暗物质的特征，并检查它们是否与我们的早期宇宙理论相一致。分析数据是困难的，因为产生的许多粒子是更基本的粒子夸克的复杂团块，它们被强核力粘在一起。事实上，在大型强子对撞机上碰撞的最初质子是这样的。没有简单的方法来计算在这个“粘在一起”的过程中发生了什么，必须开发特殊的数学技术。这是我们在剑桥工作的一个重要重点。在我们的一些理论中，方程有许多隐藏的对称性，可以被梳理出来并加以利用;在另一些理论中，我们可以用一组离散点来近似时空，并在计算机上进行计算。我们想解决的一个问题是质子和中子本身如何在原子核中粘在一起。我们正在发展一种近似理论，在这种理论中，原子核的形状可以作为方程的解而得到。即使希格斯玻色子和超对称性被发现，粒子物理学中仍有许多未解之谜。例如，引力如何融入粒子物理学？在数学上，引力导致粒子理论在远小于质子大小的距离尺度上的不一致，这可能是因为时空本身通过量子涨落变得模糊。弦理论，以及它微小的卷曲的额外维度，为我们提供了一个数学上一致的解决方案的最大希望。该小组的大部分工作将用于进一步研究、检验和发展弦理论的数学结构。弦理论部分预测了LHC可能发现的新粒子。当人们试图建立包含我们已知粒子的显式弦理论时，这些预言就出现了。已知的粒子对卷曲的额外维度的形状施加了限制，这在数学上非常有趣。我们将考虑所有这些对我们理解整个宇宙的影响。例如，弦理论对于宇宙的“反弹”解有着有趣的结果，在那里它不断地坍缩和重新膨胀，通过量子力学平滑了大爆炸奇点。这一点，以及其他的宇宙模型，将通过对大爆炸余辉的测量--宇宙微波天空--以及对宇宙中“暗能量”和“暗物质”数量的日益准确的测量来进行测试。

The Large Hadron Collider (LHC) will start operating this year, colliding protons at much greater energies than ever achieved before. Through Einstein's famous equation $E=mc^2$, this greater energy can be turned into the masses of heavier particles than ever seen before. Hints from experiments a decade ago suggest that at least the Higgs particle, or something very like it, should be produced at LHC. The Cambridge group are developing techniques to extract the few Higgs particles' signature decays from the billions upon billions of other particles produced. We shall study how to extract properties of the Higgs particle from the data, including its spin, which theorists have predicted to be zero. Building accurate computer simulations of the collisions and subsequent decays is one important activity. In our current theories, the Higgs particle is responsible for the origin of the mass of all other fundamental particles, but it is a puzzle why the Higgs is not far heavier still. Speculative ideas such as supersymmetry, or extra space dimensions, can solve the puzzle, but we cannot believe in these unless their effects are unambiguously measured. One possible effect is the production of particles of dark matter at the LHC, whose signature is 'missing energy' in the particle collision. We shall need to investigate any dark matter signatures and check whether they are compatible with our theories of the early universe. Analysing data is difficult because many of the particles produced are complicated lumps of more fundamental particles, the quarks, stuck together by the strong nuclear force. Indeed, the initial protons colliding at the LHC are like this. There is no easy way to calculate what happens in this 'sticking together' process, and special mathematical techniques have to be developed. This is an important focus of our work at Cambridge. In some of our theories the equations have a lot of hidden symmetry that can be teased-out and exploited; in others we can approximate space-time by a set of discrete points and do calculations on computers. One problem we want to solve is how protons and neutrons themselves stick together in the nuclei of atoms. We are developing approximate theories in which the shapes of nuclei come out as solutions of the equations. Even if the Higgs and supersymmetry were to be discovered, there remain many mysteries in particle physics. For example, how does gravity fit into particle physics? Mathematically, gravity leads to inconsistencies in particle theories at distance scales much smaller than the size of a proton, possibly because space-time itself becomes fuzzy through quantum fluctuations. String theory, with its tiny curled-up extra dimensions, offers our best hope of a mathematically consistent solution. Much of the group's effort will go into further investigating, checking and developing the mathematical structure of string theory. String theories partly predict what new particles might be discovered at the LHC. These predictions emerge when one tries to build explicit stringy theories incorporating the particles we already know about. Known particles place constraints on the shape of the curled-up extra dimensions, and this is very interesting mathematically. We shall be considering the effects of all this on our understanding of the universe as a whole. For instance, string theory has interesting consequences for 'bounce' solutions of the universe where it continually collapses and re-expands, smoothing out the big bang singularity through quantum mechanics. This, and alternative models of the universe, will be tested against measurements of the after-glow of the big bang -- the Cosmic Microwave sky -- and the increasingly accurate measurements of the amount of 'dark energy' and 'dark matter' in the universe.