Spectator Tagging Experiments to Understand the Structure of Matter

我们如何测量一个亚原子粒子，它只存在了非常短的时间，甚至在我们可以检测到它之前就消失了？这项研究旨在设计创新技术，使我们能够做到这一点，突破我们对物质结构的重要知识的现有限制。我们观察原子核的内部动力学的能力即将发生变化，原子核是我们宇宙的主要组成部分之一。可见宇宙99.9%的质量存在于原子核中，原子核由质子和中子(核子)组成。核子本身具有复合结构，由部子(夸克和胶子)组成。强子是包括核子在内的亚原子粒子，它们通过基本的强相互作用结合在一起并相互作用。这种相互作用在许多核过程中是至关重要的，例如燃烧的恒星中的连锁反应，或用于能量的核反应堆。量子色动力学(QCD)是描述强相互作用的理论。理解这一点将是回答核物理学中一些最紧迫的问题的关键，比如“为什么夸克在自然界中不能自由存在，为什么它们被限制在强子内部？”或者“强子质量的起源是什么？”要理解QCD，我们必须了解强子的部分子结构。核子内部的部分子排列是不断变化的，在核子内部不断地产生和销毁强子。大约20%的时间，核子以一种虚拟的(非常短暂的)状态存在，在这种状态下，它伴随着介子。介子是最简单的束缚强子，由一个夸克和一个反夸克组成。它们是核子之间的力调解器，将原子核保持在一起，是QCD有价值的虚拟实验室。尽管关于它们的结构在理论上取得了重大进展，但实验数据极其稀少。通过这项研究，英国科学家将领导旨在从这些虚拟介子核子云中散射高能电子的实验。这将揭示核子难以捉摸的介子含量，并提供了解两种不同介子--介子和介子的复合结构的途径。这些介子的夸克/反夸克含量不同。虚介子云本身的散射量是无法测量的--散射过程中的介子碎片。为了成功地选择正确的散射事件，必须通过测量与散射电子一致的相互作用产生的相应核子反冲来推断它的存在。如果散射发生在介子云上，核子将以特定的能量沿某些预期的方向运动。这是观众标记，并提供了访问在自然界中不易找到的目标的途径。这项研究将优化反冲核子探测器的设计，并在实验中使用它。探测器将装满气体。当带电粒子穿过它时，就会产生一个信号。这些信号将被快速电子设备不断记录下来，以确定粒子的方向和能量。这将是一项突破性的技术。它必须能够承受和解开其活跃体积内极高速率的粒子(每秒数百万个粒子)。实验将在最近升级的杰斐逊实验室(美国)进行，提供前所未有的高强度电子束，并为这些罕见的散射过程开辟了前沿。这些实验将为进一步研究强子结构提供一个通向未来观众标记实验的大门。在这一领域开发的技术在一些有益于社会的应用中也具有潜在的影响。例如，在医学物理或核废物退役方面，尖端粒子探测器是非常有用的，以及研究其实施所需的详细计算机模拟的知识。

How can we measure a sub-atomic particle which only comes into existence for an exceptionally short time and disappears before we can even detect it? This research aims to design innovative technology enabling us to do this, to push beyond current limits in our vital knowledge of the structure of matter.We are on the brink of a transformation in our capabilities to observe the inner-dynamics of the nucleus, one of the main building blocks of our universe. 99.9% of the mass of the visible universe resides in atomic nuclei, which are composed of protons and neutrons (nucleons). Nucleons themselves have a composite structure, consisting of partons (quarks and gluons).Hadrons are sub-atomic particles, including nucleons, which are bound together and interact by the fundamental strong interaction. This interaction is vital in many nuclear processes, for example chain reactions in burning stars, or nuclear reactors for energy. Quantum chromodynamics (QCD) is the theory describing the strong interaction. Understanding it will be key to answering some of the most pressing questions in nuclear physics, like "why don't quarks exist freely in Nature and why are they confined inside hadrons?" or "what is the origin of a hadron's mass?".To understand QCD we must understand the structure of hadrons in terms of their partons. The arrangement of partons inside the nucleon is constantly changing, with hadrons continuously being created and destroyed inside it. About 20% of the time the nucleon exists in a virtual (very short-lived) state in which it is accompanied by a meson. Mesons are the simplest bound hadrons, comprising one quark and one anti-quark. They are the force mediators between nucleons, holding nuclei together, and are valuable virtual laboratories for QCD. Despite significant theoretical advances concerning their structure, experimental data are extremely sparse.Through this research, UK scientists will lead experiments aiming to scatter high energy electrons from these virtual meson "clouds" of nucleons. This will shed light on the nucleon's elusive meson content and provide access to the composite structure of two different mesons - the pion and kaon. These mesons differ in their quark/anti-quark contents.Scattering from the virtual meson cloud itself cannot be measured - the meson fragments during the scattering. To successfully select the correct scattering events, its existence must be inferred by measuring the corresponding nucleon recoiling from the interaction, in coincidence with the scattered electron. If the scattering took place from the meson cloud, the nucleon will travel in certain expected directions, with specific energies. This is spectator tagging and offers access to targets not easily found in Nature.This research will optimise the design of the detector for the recoiling nucleons and operate it in the experiments. The detector will be filled with gas. When charged particles pass through it, a signal is created. The signals will be constantly recorded by fast electronics, to determine the directions and energies of the particles. This will be a ground breaking piece of technology. It must be capable of withstanding, and disentangling, extremely high rates of particles within its active volume (several millions of particles per second).The experiments will be performed at the recently upgraded Jefferson Lab (USA), offering an unprecedented high intensity electron beam and opening the frontier to these rare scattering processes. The experiments will provide a gateway to a future landscape of spectator tagging experiments to study hadronic structure further. Techniques developed in this field also have potential impact in several applications which benefit society. For example in medical physics or nuclear waste decommissioning, where cutting edge particle detectors are instrumental, as well as knowledge of the detailed computer simulations required to study their implementation.