Neglected Symmetry: Parity doubling in QCD
The possibility that excited hadrons occur in nearly degenerate pairs of opposite parity -- a phenomenon known as parity doubling -- has been considered from time totime since the 1960's. Early arguments were based on Regge theory. There havebeen speculations that dynamical symmetries, not appearing in the underlying QCD Lagrangian, might be responsible for this phenomenon. Most recently it has beensuggested that parity doubling can be explained by "restoration" of the underlying SU(2)_L x SU(2)_R chiral symmetry of QCD at high energies. In this talk I will analyze parity doubling among baryons made of u, d, and s quarks. I will attempt to address three questions: First, what is the experimental evidence for parity doubling? Second, could asymmetry of the underlying QCD Lagrangian be responsible for parity doubling, or could it be an emergent phenomenon? And third, even if the origins of parity doubling lie elsewhere SU(2)_L x SU(2)_R is of fundamental importance of QCD, and is widely discussed in connection with spectroscopy. What, if any, are its implications for the classification of hadrons?