Restoration of chiral and U(1)A symmetries in excited hadrons.

1 Restoration of chiral and U (1) A symmetries in excited hadrons. The effective restoration of SU(2) L × SU(2) R and U(1) A chiral symmetries of QCD in excited hadrons is reviewed. While the low-lying hadron spectrum is mostly shaped by the spontaneous breaking of chiral symmetry, in the high-lying hadrons the role of the quark con-densate of the vacuum becomes negligible and the chiral symmetry is effectively restored. This implies that the mass generation mechanisms in the low-and high-lying hadrons are essentially different. The fundamental origin of this phenomenon is a suppression of quark quantum loop effects in high-lying hadrons relative to the classical contributions that preserve both chiral and U(1) A symmetries. Microscopically the chiral symmetry breaking is induced by the dynamical Lorentz-scalar mass of quarks due to their coupling with the quark condensate of the vacuum. This mass is strongly momentum-dependent, however, and vanishes in the high-lying hadrons where the typical momentum of valence quarks is large. This physics is illustrated within the solvable chirally-symmetric and confining model. Effective Lagrangians for the approximate chiral multiplets at the hadron level are constructed which can be used as phenomenological effective field theories in the effective chiral restoration regime. Different ramifications and implications of the effective chiral restoration for the string description of excited hadrons, the decoupling of excited hadrons from the Goldstone bosons, the glueball-quark-antiquark mixing and the OZI rule violations are discussed. Contents 1. Introduction 2. Chiral symmetry of QCD 3. Effective chiral and U(1) A symmetry restoration in excited hadrons by definition 4. Why should one expect a restoration of chiral and U(1) A symmetries in highly excited hadrons 5. Simple quantum-mechanical example of effective symmetry restoration 6. Can the quark-hadron duality and the Operator Product Expansion predict the rate of the symmetry restoration? 7. Chiral multiplets of excited mesons 7.1 Representations of the parity-chiral group 7.2 J = 0 mesons