Chiral Physics in Mesic Atoms and Mesic Nuclei

In the contemporary hadron physics, the light pseudoscalar mesons (π, K, η) are recognized as the Nambu-Goldstone bosons associated with the spontaneous breaking of the QCD chiral symmetry. In real world, these mesons, together with heavier η′(958) meson, show the involved mass spectrum, which are believed to be explained by the explicit flavor SU(3) breaking due to current quark masses and the breaking of the axial UA(1) symmetry at the quantum level referred as the UA(1) anomaly.1),2) One of the most important subjects in hadron physics at present is to reveal the origin of the hadron mass spectra and to find out the quantitative description of hadron physics from QCD.3) Recently, there are several very important developments for the studies of the spontaneous breaking of chiral symmetry and its partial restoration at finite density. To obtain deeper insights on the in-medium behavior of spontaneous chiral symmetry breaking, the hadronic systems, such as pionic atoms,4)–6) η-mesic nuclei7)–10) and ω-mesic nuclei,7),8),11)–13) have been investigated in both of theoretical and experimental aspects. Especially, after a series of deeply bound pionic atom experiments,14),15) Suzuki et al. reported the quantitative determination of pion decay constant fπ in medium from the deeply bound pionic states in Sn isotopes5),16) and stimulated many active researches of the partial restoration of chiral symmetry at finite density.4),6),17)–19) In this paper, we summarize the current interests of the chiral dynamics in mesic atoms and mesic nuclei.