We review recent advances in the understanding of the Quantum Chromodynamics (QCD) transition and its nature, paying special attention to the analysis of chiral symmetry restoration within different approaches based on effective theories. After presenting some of the main aspects of the current knowledge of the phase diagram from the theoretical, experimental and lattice sides, we discuss some recent problems where approaches relying on effective theories have been particularly useful. In particular, the combination of ideas such as Chiral Perturbation Theory, unitarity and Ward Identities allows us to describe successfully several observables of interest. This is particularly relevant for quantities expected to be dominated by the light meson components of the hadron gas such as the scalar and topological susceptibilities. In addition, ward identities and effective Lagrangians provide systematic results regarding chiral and $U(1)_A$ partner degeneration properties which are of great importance for the interplay between those two transitions and the nature of chiral symmetry restoration. Special attention is paid to the connection of this theoretical framework with lattice simulations.

中文翻译：

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关于有效理论和 QCD 过渡的方面

我们回顾了对量子色动力学 (QCD) 跃迁及其性质的理解的最新进展，特别关注基于有效理论的不同方法中手征对称性恢复的分析。在从理论、实验和晶格方面介绍了当前相图知识的一些主要方面之后，我们讨论了一些最近的问题，其中依赖有效理论的方法特别有用。特别是，手性微扰理论、幺正性和病房身份等思想的结合使我们能够成功地描述几个感兴趣的可观察量。这对于预计由强子气体的轻介子成分（例如标量和拓扑磁化率）支配的量尤其相关。此外，病房身份和有效拉格朗日函数提供了关于手性和 $U(1)_A$ 合作伙伴退化特性的系统结果，这对于这两种转变之间的相互作用和手性对称性恢复的性质非常重要。特别注意该理论框架与晶格模拟的联系。