Have physicists finally discovered glueballs? New evidence points to yes.
Have physicists finally discovered glueballs? New evidence points to yes.
物理学家终于发现“胶球”了吗？最新证据指向肯定的答案。
Physicists with the Beijing Spectrometer III (BES III) experiment have uncovered convincing new evidence of the existence of so-called glueballs, an elusive composite particle made entirely of gluons predicted by quantum theory. The results appeared in a preprint posted to arXiv last month and were also presented last week at the International Conference on High Energy Physics (ICHEP).
北京谱仪III（BES III）实验的物理学家们发现了关于“胶球”（glueballs）存在的令人信服的新证据。胶球是一种难以捉摸的复合粒子，完全由量子理论预言的胶子组成。研究结果已于上个月发布在预印本网站 arXiv 上，并于上周在国际高能物理会议（ICHEP）上进行了展示。
All the stuff we see around us is made up of quarks held together by gluons (carriers of the nuclear strong force) to form protons and neutrons, which comprise the core of every single atom. The Higgs boson, discovered in 2012 after decades of searching, was widely touted as the final missing piece of the Standard Model of Particle Physics. But there are still plenty of unanswered questions, including whether or not glueballs really exist. They should, if the Standard Model is correct; they’re a direct prediction of quantum chromodynamics, i.e., the theory of the strong nuclear force. There should even be several kinds of glueballs.
我们周围的一切物质都是由夸克组成的，夸克通过胶子（强核力的载体）结合在一起，形成了构成每个原子核心的质子和中子。希格斯玻色子在经过几十年的搜寻后于2012年被发现，它被广泛认为是粒子物理标准模型的最后一块拼图。但仍有许多未解之谜，包括胶球是否真的存在。如果标准模型是正确的，那么胶球就应该存在；它们是量子色动力学（即强核力理论）的直接预言。甚至应该存在多种类型的胶球。
As Matthew Francis wrote for Ars in 2015: Just like the Higgs boson, glueballs are part of the reason that matter has mass. The Higgs boson is a manifestation of the “Higgs field,” which is present throughout the Universe. Quarks, electrons, and other fundamental particles would be mass-free in a Higgsless cosmos, but when they interact with that field, they pick up mass. In contrast, most of the mass of protons and neutrons doesn’t come from quarks; it comes from the “glue” holding them together. Gluons are the reason for that glue (they are named “glue-ons,” after all). Though they don’t have mass, the energy involved in binding everything together inside a proton is huge, and a lot of that energy takes the form of mass thanks to E=mc2. Without gluons, protons wouldn’t exist, much less be as massive as they are.
正如马修·弗朗西斯（Matthew Francis）2015年为 Ars 所写的那样：就像希格斯玻色子一样，胶球也是物质具有质量的原因之一。希格斯玻色子是遍布宇宙的“希格斯场”的表现。在没有希格斯场的宇宙中，夸克、电子和其他基本粒子将没有质量，但当它们与该场相互作用时，它们就获得了质量。相比之下，质子和中子的大部分质量并非来自夸克，而是来自将它们束缚在一起的“胶水”。胶子正是这种“胶水”的来源（毕竟它们的名字叫“glue-ons”）。虽然胶子本身没有质量，但将质子内部所有物质束缚在一起所需的能量是巨大的，根据 E=mc2，这些能量中的很大一部分以质量的形式存在。如果没有胶子，质子就不会存在，更不用说拥有现在的质量了。
But there’s another side effect: gluons stick to each other, not just to quarks. That means it could be possible to build a particle out of just gluons, with no quarks needed—that’s the glueball. There is a dizzying array of subatomic particles in the particle zoo. Of particular relevance to the hunt for glueballs is the so-called J/ψ particle discovered in 1974, a meson consisting of one charm quark and one charm antiquark. When those particles decay, they produce a lot of gluons and composite particles known as hadrons in the process, so physicists have long thought that this was the best experimental regime in which to search for glueball signatures. According to astrophysicist Ethan Siegel, for a particle to be considered a possible glueball, it must have zero spin, no electric charge, and odd parity, among other properties.
但还有一个副作用：胶子不仅会粘在夸克上，还会粘在彼此身上。这意味着有可能仅由胶子构建出一个粒子，而不需要夸克——这就是胶球。在粒子动物园中，存在着令人眼花缭乱的亚原子粒子。与寻找胶球特别相关的是1974年发现的所谓 J/ψ 粒子，这是一种由一个粲夸克和一个反粲夸克组成的介子。当这些粒子衰变时，它们会产生大量的胶子和被称为强子的复合粒子，因此物理学家长期以来一直认为，这是寻找胶球特征的最佳实验环境。据天体物理学家伊桑·西格尔（Ethan Siegel）称，一个粒子要被视为可能的胶球，它必须具备零自旋、无电荷、奇宇称等特性。
Enter the BES III experiment, an electron-positron collider specifically designed to study J/ψ particles for telltale glueball signatures. The updated collider began collecting data in 2008 and within a year had recorded over 226 million events. One of the strongest candidates for the lightest glueball state is the X(2370) particle, discovered in 2011 at BES III. As its name implies, the mass initially was measured at 2.370 GeV/C2, a bit shy of the 2.395 GeV/C2 predicted by lattice QCD theory. By 2024, the number of recorded events exceeded 10 billion J/ψ particles. That made it statistically more likely that the experiment would pick up rare events and exotic states like XYZ mesons and tetraquarks, as well as a lot of X(2370) particles, so it was possible to measure the latter’s properties more accurately than ever before.
接下来是 BES III 实验，这是一个专门为研究 J/ψ 粒子以寻找胶球特征而设计的正负电子对撞机。升级后的对撞机于2008年开始收集数据，并在一年内记录了超过2.26亿次事件。最轻胶球态的最强候选者之一是 X(2370) 粒子，它于2011年在 BES III 被发现。顾名思义，其质量最初测量为 2.370 GeV/C2，略低于晶格 QCD 理论预言的 2.395 GeV/C2。到2024年，记录的事件数量超过了100亿个 J/ψ 粒子。这使得实验在统计学上更有可能捕捉到罕见事件和奇异态（如 XYZ 介子和四夸克态），以及大量的 X(2370) 粒子，从而能够比以往任何时候都更精确地测量后者的性质。
BES III physicists ended up with a predicted mass of 2.395 GeV/C2, an impressive agreement between experiment and theoretical predictions for a glueball. Its spin and parity were also consistent with QCD. It still wasn’t quite enough to unambiguously declare the discovery of the first glueball. But it was the strongest evidence yet found for the existence of glueballs—until now. These latest results looked at several previously unreported decay modes of X(2370). Analysis revealed that X(2370) is a “flavor singlet,” i.e., it’s not associated with one particular quark flavor, such as up, down or strange, a vital experimental clue in support of this being a particle comprised predominantly of gluons. (The collaboration estimates it’s comprised of about 90 percent gluons.) That means that X(2370) matches the three primary predicted properties for a glueball, which the BES III team considered a complete chain of evidence.
BES III 的物理学家最终得出的质量预测值为 2.395 GeV/C2，实验结果与胶球的理论预测达到了惊人的一致。其自旋和宇称也与 QCD 一致。这仍然不足以明确宣布发现首个胶球，但这是迄今为止发现的关于胶球存在的最强证据。最新的研究结果观察了 X(2370) 的几种此前未报道的衰变模式。分析显示，X(2370) 是一个“味单态”（flavor singlet），即它不与任何特定的夸克味（如上、下或奇异夸克）相关联，这是支持其主要由胶子组成的粒子的重要实验线索。（合作组估计它约由90%的胶子组成。）这意味着 X(2370) 符合胶球的三个主要预言特性，BES III 团队认为这构成了一个完整的证据链。
“It’s an experimental triumph,” Colin Morningstar, a particle physicist at Carnegie Mellon University who was not involved in the research, told Science. “It’s the strongest evidence yet that particles dominated by a glueball component can exist in nature.” The next step is for other groups to independently verify the results—perhaps at the proposed Super Tau-Charm Facility (STCF) in China or the Electron-Ion Collider under construction at Brookhaven National Laboratory in the US. BES III is currently the only machine devoted exclusively to hunt for gluons, so independent confirmation could take a while.
“这是一次实验上的胜利，”未参与该研究的卡内基梅隆大学粒子物理学家科林·莫宁斯塔（Colin Morningstar）告诉《科学》杂志。“这是迄今为止最强有力的证据，表明以胶球成分为主的粒子可以在自然界中存在。”下一步是让其他研究小组独立验证这些结果——可能是在中国提议建设的超级陶粲装置（STCF）或美国布鲁克海文国家实验室正在建设的电子-离子对撞机（EIC）上进行。BES III 目前是唯一专门用于搜寻胶子的机器，因此独立的确认可能还需要一段时间。