The importance of $S$-matrix unitarity in realistic meson spectroscopy is reviewed, both its historical development and more recent applications. First the effects of imposing $S$-matrix unitarity on meson resonances is demonstrated in both the elastic and the inelastic case. Then, the static quark model is revisited and its theoretical as well as phenomenological shortcomings are highlighted. A detailed account is presented of the mesons in the tables of the Particle Data Group that cannot be explained at all or only poorly in models describing mesons as pure quark-antiquark bound states. Next the earliest unitarised and coupled-channel models are revisited, followed by several examples of puzzling meson resonances and their understanding in a modern unitarised framework. Also, recent and fully unquenched lattice descriptions of such mesons are summarised. Finally, attention is paid to production processes, which require an unconventional yet related unitary approach. Proposals for further improvement are discussed.

中文翻译：

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现代介子光谱：幺正性的基本作用

回顾了 $S$-矩阵单一性在现实介子光谱中的重要性，包括其历史发展和最近的应用。首先，在弹性和非弹性情况下都证明了对介子共振施加 $S$-矩阵单一性的影响。然后，重新审视静态夸克模型，并强调其理论和现象学的缺点。粒子数据组的表格中详细说明了在将介子描述为纯夸克-反夸克束缚态的模型中根本无法解释或只能解释得很差的介子。接下来重新审视最早的单一化和耦合通道模型，然后是几个令人费解的介子共振的例子以及它们在现代单一化框架中的理解。还，总结了这些介子的最新和完全未淬灭的晶格描述。最后，关注生产过程，这需要非常规但相关的统一方法。讨论了进一步改进的建议。