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arXiv · 2604.27982

Chiral Symmetry and Its Restoration in QCD

Teiji Kunihiro
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Abstract

Chiral symmetry is an approximate symmetry of QCD in the light-quark sector. Its spontaneous breaking in the QCD vacuum explains why the pion is anomalously light and why it plays a central role in nuclear physics, including the tensor component of the nuclear force and the saturation properties of nuclei. This article introduces chirality through the Dirac equation and shows how a fermion mass mixes left- and right-handed components, in close analogy with the Bogoliubov--Valatin theory of superconductivity. We then explain spontaneous symmetry breaking, the Nambu--Goldstone theorem, and the role of the chiral condensate as an order parameter. The axial $U(1)_A$ anomaly and its consequences, especially for the $\eta'$ meson, are discussed. Chiral effective models, including Nambu--Jona-Lasinio-type models, linear sigma models with anomaly terms, and parity-doublet models for baryons, are reviewed as tools for describing hadron properties and the equation of state of dense matter. Finally, we discuss how chiral symmetry may be partially restored at finite temperature and/or baryon density, and summarize experimental probes such as deeply bound pionic atoms, dilepton production in relativistic heavy-ion collisions, $\eta'$-mesic nuclei, and baryonic observables.

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Teiji Kunihiro. 2026-04-30. Chiral Symmetry and Its Restoration in QCD. https://arxiv.org/abs/2604.27982

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