Abstract Despite decades of research, we still lack a detailed quantitative understanding of the way quantum chromodynamics (QCD) generates the spectrum of hadrons. Precise experimental studies of the hadron excitation spectrum and the dynamics of hadrons help to improve models and to test effective theories and lattice QCD simulations. In addition, QCD seems to allow hadrons beyond the three-quark and quark–antiquark configurations of the constituent-quark model. These so-called exotic hadrons contain additional constituent (anti)quarks or excited gluonic fields that contribute to the quantum numbers of the hadron. Hadron spectroscopy is currently one of the most active fields of research in hadron physics. The COMPASS experiment at the CERN SPS is studying the excitation spectrum of light mesons, which are composed of up, down, and strange quarks. The excited mesons are produced via the strong interaction, i.e. by Pomeron exchange, by scattering a 190 GeV/ c pion beam off proton or nuclear targets. On heavy nuclear targets, in addition the electromagnetic interaction contributes in the form of quasi-real photon exchange at very low four-momentum transfer squared. COMPASS has performed the most comprehensive analyses to date of isovector resonances decaying into η π , η ′ π , or π − π − π + final states. In this review, we give a general and pedagogical introduction into scattering theory and the employed partial-wave analysis techniques. We also describe novel methods developed for the high-precision COMPASS data. The COMPASS results are summarized and compared to previous measurements. In addition, we discuss possible signals for exotic mesons and conclude that COMPASS data provide solid evidence for the existence of the manifestly exotic π 1 ( 1600 ) , which has quantum numbers forbidden for a quark-model state, and of the a 1 ( 1420 ) , which does not fit into the quark-model spectrum. By isolating the contributions from quasi-real photon exchange, COMPASS has measured the radiative widths of the a 2 ( 1320 ) and, for the first time, that of the π 2 ( 1670 ) and has tested predictions of chiral perturbation theory for the process π − + γ → π − π − π + .

中文翻译：

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使用 COMPASS 的光介子光谱

摘要 尽管经过数十年的研究，我们仍然缺乏对量子色动力学 (QCD) 产生强子光谱的方式的详细定量理解。对强子激发光谱和强子动力学的精确实验研究有助于改进模型并测试有效的理论和晶格 QCD 模拟。此外，QCD 似乎允许强子超出成分夸克模型的三夸克和夸克-反夸克配置。这些所谓的奇异强子包含额外的成分（反）夸克或激发的胶子场，它们有助于强子的量子数。强子光谱是目前强子物理学中最活跃的研究领域之一。CERN SPS 的 COMPASS 实验正在研究光介子的激发光谱，它由上、下、和奇怪的夸克。激发介子是通过强相互作用产生的，即通过波梅隆交换，通过从质子或核目标散射 190 GeV/c π 离子束。此外，在重核目标上，电磁相互作用以非常低的四动量转移平方的准真实光子交换的形式起作用。COMPASS 对衰减到 η π 、η ′ π 或 π − π − π + 最终状态的等向量共振进行了迄今为止最全面的分析。在这篇综述中，我们对散射理论和所采用的分波分析技术进行了一般性和教学性的介绍。我们还描述了为高精度 COMPASS 数据开发的新方法。总结了 COMPASS 结果并与之前的测量结果进行了比较。此外，我们讨论了奇异介子的可能信号，并得出结论，COMPASS 数据为明显奇异的 π 1 (1600) 和 a 1 (1420) 的存在提供了确凿的证据，该 π 1 (1600) 具有禁止用于夸克模型状态的量子数不适合夸克模型谱。通过隔离准实光子交换的贡献，COMPASS 测量了 a 2 (1320) 的辐射宽度，并首次测量了 π 2 (1670) 的辐射宽度，并测试了手征扰动理论对该过程的预测π − + γ → π − π − π + 。