Search for squarks and gluinos at the ATLAS experiment

粒子物理学的目的是了解物质组成的性质以及将它们结合在一起的力。在上个世纪的早期，我们的知识从原子的概念发展到原子核及其组成部分质子和中子的发现。后来人们发现质子和中子也有一个由胶子连接在一起的夸克子结构。一张图片出现了6种不同的夸克，上和下，魅力和奇怪，顶部和底部（以及它们的反粒子），排列成3个家族，以及4种不同的力，强核力，电磁力，弱核力和引力。在轻子中也发现了类似的结构，轻子是不通过强力相互作用的粒子：电子、μ子和τ子以及它们相应的中微子。粒子物理学的标准模型（SM）描述了这些粒子和力如何相互作用，但忽略了引力。虽然高精度的实验已经验证了它的预测，SM通常被认为是一个低能量有效的理论的几个开放的问题。除此之外，它没有解释粒子质量的起源，希格斯玻色子，产生基本粒子质量所必需的假设粒子，尚未被观察到，它自己的质量也没有确定。人们已经提出了几种理论方案来描述极高能量下的物理学，其中超对称（SUSY）和额外维度（艾德）是最引人注目的。在绝大多数情况下，这些场景预见了新粒子的存在。例如，在受超对称理论启发的模型中，通常的粒子谱至少是两倍，每个粒子都有一个自旋相差半个单位的“超级伙伴”，质量可能要重得多。这解决了SM中保持希格斯质量稳定的困难之一。探索“超越标准模型”领域的最佳方法是研究物质的基本成分在极高能量下的碰撞，达到TeV尺度或更高。为了实现这一目标，高能物理界设计了最强大的加速器：大型强子对撞机（LHC）。在欧洲核子研究中心建造的LHC将以前所未有的能量碰撞质子。ATLAS是LHC的两个通用实验之一，它将试图找到未知粒子的指纹，这些粒子可能表现为共振或隐藏在大型SM背景过程下的各种复杂签名。我提出的计划的主要目的是在LHC开启的最早阶段用ATLAS探测器发现新的物理学，同时进行SM过程的测量，为未来5年更详细的长期搜索做准备。精确的超对称测量不仅本身很重要，而且也是寻找超对称粒子的不可避免的背景。此外，它们还提供了优化检测器校准的基本手段。利用从这些研究中获得的探测器性能方面的知识，我将在包括强子、轻子和大的不平衡横动量喷流的末态事件中搜索夸克和胶子的超伙伴--夸克和胶子。如果超对称性被发现，我将测量夸克和胶子衰变链的运动学，并评估它们的基本参数。LHC时代的开始是粒子物理学的一个里程碑，必须有一个明确的分析搜索策略，以最大限度地挖掘发现新物理的潜力。我的研究将很大程度上受益于我在Tevatron对撞机上获得的经验和利物浦小组的专业知识。这对我们的领域来说是一个激动人心的时刻。我们有独特的能力来探索能源前沿的未知领域，我渴望成为ATLAS物理计划的最前沿。

The aim of Particle Physics is to understand the nature of the constituents of matter and the forces that bind them together. In the early part of the last century our knowledge grew from concepts of atoms to the discovery of the nucleus and its constituent, protons and neutrons. Later it was discovered that the protons and neutrons also had a sub-structure of quarks held together by gluons. A picture emerged of 6 different quarks, up and down, charm and strange, top and bottom (and their antiparticles), arranged into 3 families, and 4 different forces, strong nuclear force, electromagnetic force, weak nuclear force and gravity. A similar structure was found among leptons, the particles that do not interact via the strong force: electron, muon and tau and their corresponding neutrinos. The Standard Model (SM) of particle physics describes how these particles and forces interact, but ignores the gravitational force. Although high-precision experiments have verified its predictions, the SM is often regarded as a low energy effective theory for several open questions. Among others, it does not explain the origin of particle masses and the Higgs boson, the postulated particle necessary to generate elementary particle mass, has not been observed yet and its own mass is unspecified. Several theoretical scenarios have been proposed to describe physics at very high energies, of which SuperSymmetry (SUSY) and Extra-Dimension (ED) are the most compelling ones. In the large majority of cases, these scenarios foresee the existence of new particles. In SUSY inspired models, for example, the usual particle spectrum is at least doubled and every particle has a 'superpartner' differing in spin by a half unit, possibly much heavier in mass. This solves one of the difficulties in the SM of keeping the Higgs mass stable. The best way to explore the 'Beyond Standard Model' territory is to study collisions between fundamental constituents of matter at extremely high energies, to the TeV scale and beyond. To accomplish this, the High Energy Physics community has designed the most powerful accelerator: the Large Hadron Collider (LHC). Built at CERN, the LHC will collide protons at unprecedented energies. ATLAS, one of the two general purpose experiments at the LHC, will try to find the fingerprints of unknown particles, which might manifest themselves as resonances or through various complex signatures hidden underneath large SM background processes. The primary aim of my proposed programme is to discover new physics with the ATLAS detector at the earliest stage of the LHC turn-on, while simultaneously performing measurements of SM processes in preparation for more detailed longer term searches over the next 5 years. Precision SM measurements are not only important in their own right but also constitute unavoidable backgrounds to searches for SUSY particles. Furthermore, they provide the essential means to optimize the calibration of the detector. Exploiting the knowledge acquired from these studies in terms of detector performances, I shall carry out searches for squarks and gluinos, the superpartner of quarks and gluons, in final state events including jets of hadrons, leptons and large unbalanced transverse momentum. If SUSY is discovered, I will measure the kinematics of squark and gluino decay chains and evaluate their fundamental parameters. The beginning of the LHC era represents a milestone in particle physics and it is imperative to have a well-defined analysis search strategy in the aim of maximizing the potential to find new physics. My research will largely benefit from the combination of the experience I gained at the Tevatron collider and the Liverpool group expertise. This is an exciting time for our field. We have the unique ability to explore the uncharted territory at the energy frontier, and I am eager to be at the forefront of the ATLAS physics programme.