Universal Fluctuations in Dirac Spectra

In these two lectures given at the 1997 Zakopane workshop on "New Developments in Quantum Field Theory" we review recent results on universal fluctuations in QCD Dirac spectra. We start the first lecture with a review of some general properties of Dirac spectra. It will be argued that there is an intimate relation between chiral symmetry breaking and correlations of Dirac eigenvalues. In particular, we will focus on the microscopic spectral density density, i.e. the spectral density near zero virtuality on the scale of a typical level spacing. The relation with Leutwyler-Smilga sum-rules will be discussed. Standard methods for the statistical analysis of quantum spectra will be reviewed. Recent results on the application of Random Matrix Theory to spectra of 'complex' systems will be summarized. This leads to the introduction of a chiral Random Matrix Theory (chRMT) with the global symmetries of the QCD partition function. In the second lecture the chiral random matrix model will be compared to QCD and some of its properties will be discussed. Our central conjecture is that correlations of QCD Dirac spectra are described by chRMT. We will review recent results showing that the microscopic spectral density and eigenvalue correlations near zero virtuality are strongly universal. Lattice QCD results for the microscopic spectral density and for correlations in the bulk of the spectrum will be presented. In all cases that have been considered, the correlations are in perfect agreement with chRMT. We will end the second lecture with a review of chiral Random Matrix Theory at nonzero chemical potential. New features of spectral universality in nonhermitean matrices will be discussed.