Beyond the Conventional Quark Model: Theoretical Studies of QCD Exotics

超越传统夸克模型：QCD 奇异物质的理论研究

• 批准号: SAPIN-2021-00024
• 负责人: Steele, Thomas
• 金额: $ 2.49万
• 依托单位: University of Saskatchewan
• 依托单位国家: 加拿大
• 项目类别: Subatomic Physics Envelope - Individual
• 财政年份: 2021
• 资助国家: 加拿大
• 起止时间: 2021-01-01 至 2022-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=743101
• 关键词: Beyond; Conventional; Quark; Model; Theoretical

The conventional quark model classifies mesons as quark-antiquark ($q\bar q$) states and baryons as $qqq$ three-quark states. Although the conventional quark model represents the simplest combinations of quarks that can form colour singlets, there is a rich range of other colour-singlet possibilities such as gluonium (consisting entirely of gluons), hybrids (a conventional hadron combined with gluonic content), and multiquark states such as four-quark $q\bar q q\bar q$ mesons and pentaquark $qqq\bar q q$ baryons. These configurations that lie beyond the conventional quark model are collectively known as exotic hadrons. Until 2003, evidence for exotic hadrons was limited to hints of gluonium and a hybrid within the light mesons. However, the 2003 discovery of the X(3872) marked the beginning of a new era in hadron spectroscopy with the discovery of a multitude of XYZ mesons that cannot be described within the conventional quark model.  The long-term passion of my research program is to advance our understanding of exotic hadrons from the theoretical perspective of quantum chromodynamics (QCD): the fundamental theory of the strong interactions. Despite intense theoretical and experimental activity, the internal structure of the XYZ mesons remains an open question. Scenarios for four-quark states include meson-meson molecules, diquark-antidiquark clusters binding into tetraquarks, and hadro-quarkonium with a heavy quark-antiquark core and a cloud of light quarks. These distinct forms of quark internal clustering are expected to manifest themselves in hadronic properties. One of the key themes in the proposed research is to develop and use new QCD sum-rule operators consisting of meson or diquark operators at separate spacetime points to model the internal molecular or tetraquark structure. The goal is to determine the effect of the internal structure on hadronic properties to gain new insights on possible internal configurations. Another main theme of the proposed research is to advance the precision of our theoretical calculations for exotic hadrons to ensure that important physics insights are not obscured by theoretical uncertainties. The vast majority of QCD sum-rule studies of exotics have been performed at leading order, so the proposed research will examine higher-loop effects on the mass spectrum and decays in a variety of exotic systems including hybrid mesons, multiquark states, and hybrid baryons. Mixed interpretations involving hybrid, quark-antiquark, and multi-quark components will be an area of investigation. My research program has pushed the boundaries of QCD sum-rule analytic calculations for exotic hadrons, and the next frontier is the use of sector decomposition methods for numerical loop-integration techniques implemented in pySecDec. Combined with novel theoretical approaches, the proposed research will pioneer the use of these numerical methods in QCD sum-rule studies of exotic hadrons.

传统的夸克模型将介子分为夸克-反夸克（$q\bar q$）态，重子分为三夸克态。虽然传统的夸克模型代表了可以形成色单态的最简单的夸克组合，但还有很多其他的色单态可能性，例如胶子（完全由胶子组成），混合体（传统的强子与胶子的结合），以及多夸克态，例如四夸克$q\bar q q\bar q$介子和五夸克$qqq\bar q q$重子。这些超越传统夸克模型的组态统称为奇异强子。直到2003年，奇异强子的证据还仅限于胶子和轻介子中的混合体的迹象。然而，2003年X（3872）的发现标志着强子光谱学新时代的开始，发现了大量无法在传统夸克模型中描述的XYZ介子。 我的研究计划的长期激情是从量子色动力学（QCD）的理论角度推进我们对奇异强子的理解：强相互作用的基本理论。尽管理论和实验活动非常激烈，XYZ介子的内部结构仍然是一个悬而未决的问题。四夸克态的场景包括介子-介子分子、双夸克-反双夸克团簇结合成四夸克、重夸克-反夸克核和轻夸克云的强子夸克偶素。这些不同形式的夸克内部成团预计将在强子性质中表现出来。在拟议的研究中的一个关键主题是开发和使用新的QCD求和规则运营商组成的介子或双夸克运营商在不同的时空点来模拟内部分子或四夸克结构。我们的目标是确定强子性质的内部结构的影响，以获得新的见解可能的内部配置。拟议研究的另一个主题是提高我们对奇异强子的理论计算的精度，以确保重要的物理学见解不会被理论不确定性所掩盖。绝大多数奇异粒子的QCD求和规则研究都是在领先阶进行的，因此拟议的研究将研究高圈对各种奇异系统（包括混合介子，多夸克态和混合重子）的质量谱和衰变的影响。混合解释涉及混合，夸克-反夸克，和多夸克组件将是一个调查领域。我的研究计划推动了奇异强子的QCD求和规则分析计算的边界，下一个前沿是使用扇区分解方法在pySecDec中实现的数值循环积分技术。结合新的理论方法，所提出的研究将开拓使用这些数值方法在奇异强子的QCD求和规则的研究。