QCD spectral sum rules and spontaneously broken chiral symmetry

Abstract

The gap Delta = 4 pi f_pi ~ 1.2 GeV of spontaneous chiral symmetry breaking is introduced as a scale delineating resonance and continuum regions in the QCD spectral sum rules for vector mesons. Basic current algebra results are easily recovered, and accurate sum rules for the lower moments of the spectral distributions are derived. The in-medium scaling of vector meson masses finds a straightforward interpretation, at least in the narrow width limit.