Ideas in Nonperturbative QCD

Abstract

The structure of the hadron spectrum is discussed in connection with the main phenomena of nonperturbative QCD: confinement and chiral symmetry breaking (CSB). For the higher part of the spectrum ($M \ge 2 GeV$) spin and chiral effects are unimportant; spectrum of $q\bar{q}$ system is described by an effective Hamiltonian deduced from QCD. The Hamiltonian reduces to relativistic quark potential model or to the open string model in two opposite limits. Hybrids are shown to appear naturally in theory and enter the multiplets which are compared to experiment and bosonic string theory. The phenomenon of conspiracy of the spectrum of radial excited states producing operator product expansion is discussed. The lower part of the spectrum $(M< 2 GeV)$ is influenced by spin and chiral effects. CSB and the chiral quark mass are deduced for the vacuum containing instantons and confining background. Main points are summarized in conclusion.