ALICE Upgrade 2 (Silicon Physicist post)

量子色动力学（QCD）是关于强作用力的理论，在过去的半个世纪里，对其性质的理解一直是世界范围内研究的焦点。QCD构成了标准模型的主要部分，对它的理解对于产生一个描述物质属性和自然基本力的整体理论至关重要，包括一个解释宇宙如何从大爆炸演化而来的大统一理论。我们研究的主要目的是利用超相对论性重离子相互作用来研究极端能量密度下的QCD。理论模型预测，在非常极端的高能量密度条件下，被限制在正常核物质中的夸克将被释放，核物质将经历一个相变，进入一个由自由夸克和胶子组成的热的、密集的等离子体，称为夸克-胶子等离子体（QGP）。据认为，这种物质状态会一直存在到大爆炸后的10^-5秒，在此之后，宇宙会冷却到足以形成质子和中子。这项研究的物理目的是研究这些极端条件下的强作用力，特别是探索这种奇异物质状态的性质。通过研究QGP，我们希望解决夸克约束的基本问题，以及夸克如何在强子中由于强作用力而获得大的有效质量（占原子质量的99%）。为了创造产生QGP所需的条件，我们必须以尽可能高的能量将重核碰撞在一起，产生微小的热（~10^13K）和密集的火球。由于由此产生的QGP只会持续大约10-22秒，在凝聚成数千个基本粒子之前，我们还必须使用最复杂的探测器来分析这些碰撞。大型强子对撞机在ALICE中以2.76 TeV的质心能量对撞铅离子。我们也把质子碰撞在一起，形成了一个很好的参考数据集。此外，出色的粒子识别和跟踪能力使ALICE本身成为研究质子-质子碰撞全局的理想探测器。这一研究领域在世界各地雇佣了大约2000名物理学家，爱丽丝是该领域迄今为止建造的最复杂的实验，研究质量中心能量的重离子碰撞，比最接近的竞争对手高出一个数量级。因此，它代表了这一领域研究的顶峰。ALICE合作现在正在为计划于2018年开始的第二次LHC升级做准备。这些升级是为了应对预期的铅光束亮度的增加，这将使光束强度增加一个数量级。光度升级的主要动机是通过关注在低和高横向动量下的稀有探针，以及对这些探针的中心性、事件平面、多粒子相关性等方面的多维分析，实现对QGP特性的精确、定量理解。

Quantum-ChromoDynamics (QCD) is the theory of the strong force and understanding its properties has been the focus ofintense World-wide research over the past half century. QCD forms a major part of the Standard Model and itsunderstanding is crucial in producing an overall theory to describe the properties of matter and the fundamental forces ofnature including a grand unification theory to explain how the Universe evolved from the Big Bang.The principal purpose of our research is to use ultra-relativistic heavy-ion interactions to study QCD at extreme energydensities. Theoretical models predict that under very extreme conditions of high energy densities the quarks, which areconfined in normal nuclear matter, will be freed and nuclear matter will undergo a phase transition into a hot, dense plasmaof free quarks and gluons known as a Quark-Gluon Plasma (QGP). It is thought that such a state of matter would haveexisted until about 10^-5 seconds after the Big Bang, after which time the Universe would have cooled sufficiently forprotons and neutrons to form.The physics aim of this research is to study the strong force under these extreme conditions and, in particular, explore theproperties of this exotic state of matter. By studying the QGP, we hope to address the fundamental questions of quarkconfinement and how quarks gain a large effective mass in hadrons due to the strong force (accounting for 99% of atomicmass).In order to create the conditions required to produce a QGP we must collide heavy nuclei together at the highest possibleenergies, creating tiny hot (~10^13K) and dense fireballs. As the resulting QGP will only last for about 10-22 seconds,before condensing in to thousands of elementary particles, we must also use the most sophisticated detectors to analysethese collisions.The LHC collides lead ions at centre-of-mass energies of 2.76 TeV in ALICE. We also collide protons together as this formsan excellent reference data set. Moreover, the excellent particle identification and tracking make ALICE an ideal detectorto study the global aspects of proton-proton collisions in their own right.This field of research employs about two thousand physicists around the world and ALICE is the most sophisticatedexperiment ever built in the field, studying heavy-ion collisions at centre-of-mass energies over an order of magnitudegreater than its nearest rival. It hence represents the pinnacle of research in this field.The ALICE collaboration is now preparing for the second LHC Upgrade scheduled to start in 2018. These upgrades arerequired to cope with the anticipated increase in lead beam luminosity which will increase beam intensities by an order ofmagnitude. The main motivation for the luminosity upgrade is to achieve a precise, quantitative understand of theproperties of the QGP by focusing on rare probes both at low and high transverse momenta as well as on multidimensionalanalysis of such probes with respect to centrality, event plane, multi-particle correlations, etc.