Hunt for Exotic Particles: Dibaryons

我们周围绝大多数可见物质的“质量”实际上并不是真正的质量。它来源于质子和中子组分，夸克和胶子的相互作用能。由于爱因斯坦著名的质能等价（E=mc^2），这些亚原子粒子的剧烈相互作用可以产生质量。对于地球上我们周围的物质和夜空中恒星中可见的物质来说，这些质量产生机制比最近发现的希格斯玻色子产生的质量更重要，希格斯玻色子只占原子核质量的不到1%。夸克和胶子是如何被限制在核子的束缚系统中的，而核子是我们周围原子核的组成部分，其机制还没有很好地建立起来。它们如何相互作用的基本理论（量子色动力学或QCD）已经提出了一段时间，但QCD如何在核子或其他强子的尺度上表现尚未得到很好的理解。这是现代物理学中最大的未解决的科学问题之一。基于描述夸克相互作用（特别是色约束）的QCD理论，人们期望夸克和胶子应该有许多可能的束缚态。直到最近几年，束缚系统的动物园只包含重子（qqq）和介子（qqbar）。然而，除了gg和ggg胶球态之外，四夸克系统和可能的五夸克态也不被QCD的基本原理所排除。最近发现的系统Zc（ccbar和ubar）彻底改变了这个领域。建立QCD的其他奇异野兽的竞赛正在进行，它们将为强相互作用物质的本质提供全新的约束和视角。我领导着一个最有希望的未来方向，以进一步我们对物质本质的认识，寻找二重子——神秘的六夸克态。奇异双重子态的可能性最早是由戴森在1964年提出的，就在盖尔曼发表夸克模型的半年之后。然而，这个话题在Jaffe提出所谓的h -二重子（一种udds态）之后才受到特别的关注。我最近领导了一项研究，证实了一种由6个夸克组成的粒子的存在——d*二重子：一种未知状态。这是近十年来强相互作用物质领域最大的成果之一，为我们对QCD的理解提供了一个新的窗口。这个奇异的物体有很高的角动量，所有处于基态的6个夸克的自旋都是对齐的。这种d*共振是由WASA合作研究的，主要是在我的领导下，在质子束对原子核的反应中，这是一个正在进行的项目的一部分。我希望领导一个全新的研究分支，用高能光子和电子束来研究这个物体，寻找它的姐妹粒子。这些是明确确定新的奇异粒子所需的拼图中缺失的关键部分。我还将建立新的双重子粒子是如何与光子和电子相互作用的——它的电磁特性。这将使双重子的大小受到限制，使所有6个夸克紧密包裹的真正的双重子能够与每个3个夸克的两个包裹的分子型物体区分开来。双重子物理学的新领域将对包括原始核合成、恒星动力学、暗物质搜索和夸克约束性质在内的广泛研究领域产生重大影响。

The vast majority of the "mass" of visible matter around us is in fact not really mass. It arises from the interaction energy of the components of the proton and neutron, the quarks and gluons. Because of Einstein's famous equivalence of mass and Energy (E=mc^2) the violent interactions of these subatomic particles can generate mass. For the matter around us on Earth and the matter visible in the stars of the night sky these mass generation mechanisms are much more relevant than the mass arising from the recently discovered Higgs, which accounts for less than 1% of the mass of the atomic nucleus. The mechanism of how quarks and gluons are confined into the bound systems of nucleons, which are the building blocks of the nuclei around us, is not well established. The basic theory of how they interact (Quantum Chromo Dynamics or QCD) has been proposed for some time, but how QCD manifests at the scale of the nucleon or other hadrons is not well understood. It is one of the biggest unsolved scientific problem in modern physics. Based on the QCD theory describing how quarks interact (specifically color confinement) one expects that there should be many possible bound states of quarks and gluons. Until very recent years the zoo of bound systems comprised only baryons (qqq) and mesons (qqbar). However in addition the gg and ggg glueball states, tetra-quark systems and possibly pentaquark states are not ruled out by the basic principles of QCD. The field has been revolutionized by the recently discovered system Zc(c cbar d ubar). The race is on to establish the other exotic beasts of QCD which will give a totally new constraint and perspective on the nature of strongly interacting matter. I lead one of the most promising future directions to further our knowledge of the nature of matter, the hunt for dibaryons - mysterious six-quark states.The possibility of exotic dibaryonic states was first proposed by F. Dyson in 1964, just half a year after the Gell-Mann's publication of the quark model. However, this topic received particular attention only after Jaffe's proposal of the so-called H-dibaryon, a uuddss state. I recently led research which has established the existence of a particle made from 6 quarks - the d* dibaryon: a uuuddd state. This is one of the biggest results in the field of strongly interacting matter in the past decade and provides a new window to investigate our understanding of QCD. This exotic object has a high angular momentum, with all six quarks in the ground state having their spins aligned. This d* resonance was investigated in reactions of proton beams on nuclei by the WASA collaboration, largely under my leadership, and is part of an ongoing program. I wish to lead a completely new research branch to investigate this object using beams of high energy photons and electrons to search for it's sister particles. These are the crucial missing piece of the jigsaw needed to unambiguously establish the new exotic particle. I will also establish how the new dibaryon particle interacts with the photon and the electron - it's electromagnetic properties. This will allow the size of the dibaryon to be constrained, enabling the genuine dibaryon with all 6 quarks tightly packed to be distinguished from a molecular-type object with two packages of 3 quarks each.The new field of dibaryon physics will have major impact on a wide range of research areas including primordial nucleosynthesis, star dynamics, dark matter searches and the nature of quark confinement.