Precision supersymmetry at the LHC

在过去几十年的对撞机实验的基础上，建立了所谓的基本粒子物理的标准模型，该模型包含了到目前为止观察到的所有基本粒子及其相互作用。它基于被称为“局域规范对称性”的对称性，这种对称性约束了基本粒子之间可能的相互作用。事实上，桑达尔模型的预测能力如此之强，以至于可以推断粒子的存在，这是几年后在对撞机实验中才发现的。例子包括调解弱相互作用的W玻色子和Z玻色子，其实验发现导致1984年诺贝尔奖，以及顶夸克，其质量可以在发现之前通过比较精确预测和精确测量来推断(诺贝尔奖，2000年)。然而，基本粒子物理标准模型中的一种成分，即以苏格兰物理学家彼得·希格斯最先提出的希格斯玻色子的名字命名，到目前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，对称性对理论的一致性至关重要。此外，从数学一致性的论证中，我们知道标准模型很可能只是我们还不知道的更一般理论的一个子部分。标准模型的一个非常吸引人的扩展是超对称性，它预测标准模型中的每个粒子都有一个“超对称伙伴”，由于其特殊的对称性质，可以以一种优雅的方式弥补标准模型的数学缺陷。超对称性预测了五个希格斯玻色子的存在，其中最轻的一个具有与标准模型预测的类似的性质。从前人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，最轻的希格斯玻色子必须位于一定的质量范围内，这对应着对撞机实验迄今无法达到的能量。然而，欧洲粒子物理实验室CERN目前正在建造一台质子-质子对撞机--大型强子对撞机(LHC)，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，这是物理学有史以来最大的实验努力之一。为了能够在大型强子对撞机上产生的大量“普通”粒子中探测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所预测的粒子的产生率和性质，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，我们才能毫不含糊地决定我们的哪些模型在自然界中实际实现了。在我们的研究计划中，我们将开发和提供对超对称观测的精确评估。这对大型强子对撞机实验的成功有三个方面的高度相关性。首先，从一个巨大的已知粒子“背景”中提取希格斯玻色子/新粒子的“信号”是很有用的。其次，准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，将这些精确度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(新粒子中的一个能对宇宙中的暗物质负责吗？)以及所有力量的大统一(模型参数是否显示了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的图景。

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.