Hadrons and Chiral Symmetry

Abstract

These lectures discuss the question of whether a key feature is seen in hadron spectroscopy--the near degeneracy of hadrons with different parity and/or spin. It has been conjectured that this is due to an effective restoration of chiral symmetry. The conjecture is that while these states are, of course, in the symmetry-broken (Nambu-Goldstone) phase, as one goes higher in the spectrum the states become progressively less sensitive to the dynamics of chiral symmetry breaking. These lectures discuss the current status of this conjecture. The motivations for the conjecture are discussed, as is evidence--both theoretical and experimental--in its favor. Possible alternative explanations for the data are also discussed.

Lectures originally given at the 2009 Schladming Winter School, "Fundamental Challenges of QCD". A version of these lecture notes will appear in Nucl. Phys. B (Proc. Suppl.)