Effective chiral restoration in the hadronic spectrum and QCD

Effective chiral restoration in the hadronic spectrum has been conjectured as an explanation of nearly degenerate multiplets seen in highly excited hadrons. The conjecture depends on the states being insensitive to the dynamics of spontaneous chiral symmetry breaking. A key question is whether this concept is well defined in QCD. This paper shows that it is by means of an explicit formal construction. This construction allows one to characterize this sensitivity for any observable calculable in QCD in Euclidean space via a functional integral. The construction depends on a generalization of the Banks–Casher theorem. It exploits the fact that all dynamics sensitive to spontaneous chiral symmetry breaking observables in correlation functions arise from fermion modes of zero virtuality (in the infinite volume limit), while such modes make no contribution to any of the dynamics which preserves chiral symmetry. In principle this construction can be implemented in lattice QCD. The prospect of a practical lattice implementation yielding a direct numerical test of the concept of effective chiral restoration is discussed.