Precision supersymmetry at the LHC

基于过去几十年的对撞机实验，基本粒子物理学的所谓“标准模型”已经建立起来，它包含了迄今为止观察到的所有基本粒子及其相互作用。它基于被称为“局部规范对称”的对称性，这种对称性限制了基本粒子之间可能的相互作用。事实上，标准模型的预测能力是如此强大，以至于可以推断出几年之后在对撞机实验中发现的粒子的存在。例子包括介导弱相互作用的W和Z玻色子，它们的实验发现导致了1984年的诺贝尔奖，以及顶夸克，它的质量可以在发现之前通过比较精确的预测和精确的测量来推断（诺贝尔奖2000）。然而，基本粒子物理标准模型中的一种成分，被称为“希格斯玻色子”，以苏格兰物理学家彼得·希格斯（Peter Higgs）的名字命名，希格斯玻色子是第一个提出这一理论的人，迄今为止还没有在对撞机实验中发现。它是一个非常重要的粒子，因为它解释了粒子如何在不破坏对称性的情况下拥有质量，而对称性对理论的连贯性至关重要。此外，我们从数学一致性的论证中知道，标准模型很可能只是一个我们还不知道的更普遍理论的一小部分。标准模型的一个非常吸引人的扩展是超对称，它预测了标准模型中每个粒子的“超对称伴侣”，并且由于其特殊的对称性，可以以一种优雅的方式解决标准模型的数学缺陷。超对称理论预言了五种希格斯玻色子的存在，其中最轻的一种与标准模型预测的那一种具有相似的性质。从过去和现在的对撞机实验中，结合精确的理论计算，我们可以推断出最轻的希格斯玻色子必须位于一定的质量范围内，这对应于迄今为止对撞机实验无法达到的能量。然而，一个质子-质子对撞机，“大型强子对撞机”（LHC）目前正在欧洲粒子物理研究所（CERN）建造，它可以探测希格斯玻色子无法逃脱的能量范围。大型强子对撞机将于2007年开始运行，是物理学史上最大的实验项目之一。为了能够在大型强子对撞机产生的大量“普通”粒子中探测到希格斯玻色子和超对称粒子，非常精确的理论计算是必要的。只有准确地知道不同模型（即标准模型、超对称模型的不同变体或标准模型的非超对称扩展）所预测的粒子的产生速率和性质，我们才能明确地确定我们的模型中哪一个实际上是在自然界中实现的。在我们的研究计划中，我们将开发并提供超对称观测的精确评估。这在三个方面与大型强子对撞机实验的成功高度相关。首先，从大量已知粒子的“背景”中提取希格斯玻色子/新粒子的“信号”是有用的。其次，精确的理论预测是区分解释新发现的不同模型的唯一方法。最后，将这些精确计算与实验数据进行比较，可以确定新的模型参数，测试与宇宙学的联系（其中一种新粒子是否对宇宙中的暗物质负责？）和所有力的大统一（模型参数是否显示出与大统一兼容的模式？），从而建立一个新的更完整的基本相互作用图景。

Based on collider experiments in the last decades, the so-called 'Standard Model' of elementary particle physics has been established, which incorporates all elementary particles that have been observed so far and their interactions. It is based on symmetries called 'local gauge symmetries', which constrain the possible interactions of the fundamental particles. In fact, the predictive power of the Sandard Model is so strong that the existence of particles could be inferred which have been found only years later at collider experiments. Examples include the W and Z bosons which mediate the weak interactions and whose experimental discovery lead to the Nobel Prize in 1984, and the top quark whose mass could be inferred before its discovery by comparing precision predictions with precision measurements (Nobel Prize 2000). However, one ingredient of the Standard Model of elementary particle physics, called the 'Higgs boson' after the Scottish physicist Peter Higgs who first suggested it, has not been found at collider experiments up to now. It is a very important particle as it offers an explanation how particles can have mass without destroying the symmetries which are vital for the coherence of the theory. Further, we know from arguments of mathematical consistency that the Standard Model is most likely only a subpart of a more general theory which we do not know yet. A very appealing extension of the Standard Model is Supersymmetry, which predicts a 'supersymmetric partner' to each particle in the Standard Model and, due to its special symmetry properties, can cure the mathematical deficiencies of the Standard Model in an elegant way. Supersymmetry predicts the existence of five Higgs bosons, where the lightest one has similar properties as the one predicted by the Standard Model. From previous and present collider experiments, combined with precise theoretical calculations, we can deduce that the lightest Higgs boson has to lie in a certain mass range, which corresponds to an energy that could not be reached by collider experiments so far. However, a proton-proton collider, the 'Large Hadron Collider' (LHC) is being constructed at the moment at CERN, the European Laboratory for Particle Physics, which can probe an energy range where the Higgs boson cannot escape. The LHC will start operating in 2007 and is one of the biggest experimental endeavours physics has ever seen. In order to be able to detect Higgs bosons and supersymmetric particles among the wealth of 'ordinary' particles that will be produced at the LHC, very precise theoretical calculations are necessary. Only by knowing exactly the production rates and properties of the particles which are predicted by the different models, i.e. the Standard Model, different variants of supersymmetric models or non-supersymmetric extensions of the Standard Model, we can decide unambiguously which of our models is actually realised in nature. In our research programme we will develop and provide precise evaluations of supersymmetric observables. This is of high relevance for the success of LHC experiments in three ways. First it is useful to extract Higgs boson/new particle 'signals' from a huge 'background' of known particles. Second, precise theoretical predictions are the only way to discriminate between different models explaining the new findings. Finally, comparing these precision calculations with experimental data will allow to determine the new model parameters, to test links to cosmology (can one of the new particles be responsible for the dark matter in the universe?) and grand unification of all forces (do the model parameters display a pattern that is compatible with grand unification?) and thus to establish a new and more complete picture of fundamental interactions.