Proof of Principle for CMS High-Granularity Calorimeter

粒子物理学是研究自然界的基本组成部分和控制它们相互作用的力的学科。在过去的40年里，我们发展了粒子物理学的所谓“标准模型”，它概括了我们对亚原子世界的理解。虽然它取得了惊人的成功，确实是20世纪基础物理学的最高荣耀，但我们知道它不可能是最终答案。例如，它不包含引力或解释暗物质。更直接地说，它预测了一种与质量产生有关的粒子——希格斯玻色子的存在。直到2012年欧洲核子研究中心（CERN）的大型强子对撞机（LHC）上的ATLAS和CMS合作项目发现这一标准模型的基石之前，还没有任何实验证据。希格斯粒子的发现为超越标准模型的新物理学打开了一扇窗。通过对新粒子的详细研究和寻找其他希格斯玻色子来理解这一发现的含义是世界范围内粒子物理学的最高优先事项之一。为了足够详细地研究希格斯玻色子，以及许多其他重要的物理主题，我们需要比目前更大的数据集。大型强子对撞机将进行大规模升级，以产生更大的数据集，方法是让在大型强子对撞机中加速的质子以更高的速度碰撞；这次升级被称为“高亮度LHC”（HL-LHC）。与此同时，记录这些碰撞的CMS等实验也必须升级，以应对辐射水平的增加，以及与大型强子对撞机升级相关的辐射率的增加。CMS将需要在前方区域更换其所谓的“量热计”，因为这是辐射剂量最高的地方。本文提出了一种新的热量计升级的概念，而本文提出的工作将是其中的关键部分。传统上，量热计只能测量碰撞中产生的粒子的能量，但技术的进步及其在粒子物理学中的应用，部分是由英国首创的，现在可以考虑使用高粒度量热计。这种装置还可以有效地测量粒子在量热计内的路径或轨迹。这在区分近距离颗粒方面带来了很大的优势，在传统的量热计中，这将被视为一个沉积物。高粒度量热计将由致密的吸收材料组成，这种材料会导致入射粒子失去能量，并散布着能够跟踪单个粒子通过的硅探测器。这些额外的信息对于克服Hl-LHC充满挑战的环境是非常有力的。该提案将实现该项目的两个关键方面。首先，我们将清楚地证明，这种探测器提供的单个粒子跟踪的潜在收益可以在大型强子对撞机这样的环境中完全实现。作为其中的一部分，我们将提供一个优化的探测器布局，即确定吸收器和硅传感器的最佳比例，以及优化硅传感器本身的尺寸。第二个目标是证明，即使在恶劣的大型强子对撞机环境中，从这种设备获得的额外信息也可以用于有效触发，即实时选择可能感兴趣的事件。作为其中的一部分，将这些触发算法编码到最先进的“FPGA”中是可以预见的，这将提供一个原理证明，这种方法在当前技术下是可能的。

Particle physics is the study of the fundamental building blocks of nature and the forces that govern their interactions. Over the last 40 years we have developed the so-called 'standard model' of particle physics, encapsulating our understanding of this sub-atomic world. Whilst amazingly successful, and indeed a crowning-glory of 20th century fundamental physics, we know it cannot be the final answer. For example it doesn't incorporate gravity or explain dark matter. More immediately it predicts the existence of a particle, the Higgs boson, associated with mass generation. Until the discovery by the ATLAS and CMS Collaborations at the Large Hadron Collider (LHC) at CERN in 2012 there was no experimental evidence for this corner-stone of the standard model. The discovery of a Higgs particle has opened a window onto the new physics beyond the standard model. Understanding the implications of this discovery through detailed study of the new particle and searches for other Higgs bosons is one of the highest priorities within particle physics worldwide.To study the Higgs boson in sufficient detail, and indeed for many other important physics topics, we need much larger datasets than we have so far. The LHC will undergo extensive upgrades to generate these larger data sets by colliding the protons accelerated in the LHC at even greater rates; this upgrade is referred to as the 'High-Luminosity LHC' (HL-LHC). In parallel, the experiments, such as CMS that record these collisions, must also be upgraded to cope with the increased radiation levels, and increased rates associated with the LHC upgrades.CMS will need to replace its so-called 'calorimeters' in the forward regions as this is where the radiation doses are highest. A novel concept is proposed for the calorimeter upgrade, and the work proposed here would form a key part of this. Traditionally calorimeters have only measured the energy of the particles produced in the collisions, but advances in technology and their application to particle physics, in part pioneered by the UK, now make it possible to consider high granularity calorimeters. Such devices can effectively also measure the path, or track, that a particle follows within the calorimeter. This brings large advantages in distinguishing close-by particles, which would be seen as one deposit in a traditional calorimeter. A high granularity calorimeter would consist of dense absorbing material that causes incident particles to lose energy, interspersed with silicon detectors able to track the passage of individual particles. This additional information can be very powerful in overcoming the challenging environment of the Hl-LHC. This proposal will deliver on two key aspects of this project. First we will clearly demonstrate that the potential gains of individual particle tracking that such a detector offers can be fully realised in an environment such as the LHC. As part of this we will deliver an optimised detector layout i.e. determine the optimum ratio of absorber and silicon sensors, along with optimising the size of the silicon sensors themselves. The second goal is demonstrate that the additional information available from such a device can be used, even in the harsh LHC environment, to effectively trigger, i.e. select in real time, possible events of interest. As part of this, coding these trigger algorithms into a state-of-the art 'FPGA' is foreseen and this will provide a proof-of-principle that such an approach is possible with current technology.