Birmingham Nuclear Physics Consolidated Grant 2020

我们提出的研究有两个广泛的主题，它们建立在我们世界领先的专业领域的基础上。其中第一个涉及世界上能量最高的粒子加速器大型强子对撞机上的高能核碰撞研究。爱丽丝实验的目的是研究核物质，因为在宇宙大爆炸后大约百万分之一秒的时间里，核物质就已经存在了，当时宇宙是如此之热和如此密集，以至于原子核还不存在。在其原始状态下，核物质由其等离子体状态的基本成分(夸克和胶子)组成。我们在实验中重建了这种新的物质状态，我们正在开发研究这些高能核碰撞的方法，以发现夸克-胶子等离子体的性质。这在技术上具有挑战性，该团队已经开发出一种复杂的电子触发系统来控制实验并对其完全负责。夸克-胶子等离子体具有显著的性质，如丰富的奇异夸克和近乎完美的流动性。在这项提议中，我们试图通过找到仍保留这些性质的最小一滴等离子体来确定大小是否重要。我们正在利用掠射碰撞来探索高能原子核的内部结构。我们正在观察夸克和胶子的碎片，这些碎片有时会从碰撞中散射出来，在我们的探测器中产生粒子阵雨，称为喷流，以研究等离子体内部的条件。我们还在研发基于硅像素探测器的新探测器技术，其中读出的电子设备包含在像素内。第二链从夸克延伸到原子核的规模。这里的挑战是理解强相互作用的本质是如何在核而不是亚核子尺度上发挥作用的。在这里，强大的力量非常复杂，这使得理论预测变得令人敬畏。我们的方法是对轻核进行极其精确的测量，以揭示质子和中子相对较少的系统中“核力”的各个方面。这些性质可以用来区分不同的理论，并帮助识别重要的相互作用。核力的一个表现是观察到质子和中子在较大的原子核内聚集在一起，例如形成阿尔法粒子(两个质子和两个中子)。观察这些原子核是如何解体的，就可以揭开它们的结构面纱。伯明翰小组对碳-12中的霍尔态如何分解为三个阿尔法粒子进行了世界上最灵敏的测量。(霍尔态是碳-12的一种激发形式，它决定了恒星中碳和其他元素的含量，因此详细了解它的结构非常重要。)我们建议扩展这项研究的一些系统，这些系统将提供对核分子(通过共享中子结合在一起的核物质集群)和核等现象的更深层次的洞察，这些核对核合成非常重要。最后，我们正在开发一项实验计划，利用伽马射线束通过其电磁特性非常精确地探测簇状原子核的结构。到目前为止，这个工具已经为我们提供了一些关于轻核结构的最好的见解，我们计划将这些研究扩展到高于团簇衰变能量阈值的奇异团簇态。这一计划将产生限制最先进理论的测量结果。

Our proposed research has two broad themes that build upon our world leading areas of expertise. The first of these involves the study of high-energy nuclear collisions at the Large Hadron Collider, the world's highest energy particle accelerator. The aim of the ALICE experiment is to study nuclear matter, as it would have existed about a millionth of a second after the Big Bang when the Universe was so hot and so dense that nuclei did not exist. In its primordial state nuclear matter consists of its fundamental constituents (quarks and gluons) in a plasma state. We recreate this novel state of matter in our experiment and we are developing ways of studying these high-energy nuclear collisions to discover the properties of the quark-gluon plasma. This is technically challenging and the group has developed a sophisticated electronic trigger system that controls the experiment and is entirely responsible for it. The quark-gluon plasma has remarkable properties, such as an abundance of strange quarks and near-perfect fluidity. In this proposal, we are trying to determine whether size matters by finding the smallest drop of plasma that still retains these properties. We are using grazing collisions to explore the internal structure of nuclei at high energy. And we are looking at the debris of quarks and gluons that are sometimes scattered out of the collision, producing a shower of particles in our detector known as a jet, to study the conditions inside the plasma. We are also performing R&D into new detector technologies based on silicon pixel detectors in which the readout electronics is contained within the pixel.The second strand extends beyond the quarks to the scale of nuclei. Here the challenge is to understand how the nature of the strong interaction plays out on the nuclear, rather than the sub-nucleon scale. Here the strong force is highly complex, which makes theoretical predictions formidable. Our approach is to make extremely precise measurements of light nuclei to reveal aspects of the 'nuclear force' in systems with relatively few protons and neutrons. These properties can then be used to discriminate between theories and help identify the important interactions. One manifestation of the nuclear force is the observation of clustering in which protons and neutrons clump together inside larger nuclei, for example into alpha particles (two protons and two neutrons). Watching how such nuclei fall apart enables their structure to be unveiled. The Birmingham group has made the world's most sensitive measurements of how the Hoyle state in carbon-12 falls apart into three alpha particles. (The Hoyle state is an excited form of carbon-12 which governs how much carbon and other elements are made in stars, so understanding its structure in detail is of great importance.) We propose to extend this research a number of systems that will provide a deeper insight into phenomena such as nuclear molecules (clustered of nuclear matter bound together by sharing neutrons) and nuclei important for nucleosynthesis. Finally, we are developing an experimental programme to exploit gamma-ray beams to probe with great precision the structure of clustered nuclei via their electromagnetic properties. To-date this tool has provided us with some of the best insights into the structure of light nuclei and we plan to extend these studies to exotic cluster states above the cluster-decay energy threshold. This programme will produce measurements to constrain state-of-the-art theory.