Universal Behavior in Dirac Spectra

Abstract

In these lectures we review recent results on universal fluctuations of QCD Dirac spectra and applications of Random Matrix Theory (RMT) to QCD. We review general properties of Dirac spectra and discuss the 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. The success of applications of RMT to spectra of 'complex' systems leads us to the introduction of a chiral Random Matrix Theory (chRMT) with the global symmetries of the QCD partition function. Our central conjecture is that it decribes correlations of QCD Dirac spectra. We will review recent universality proofs supporting this conjecture. Lattice QCD results for the microscopic spectral density and for correlations in the bulk of the spectrum are shown to be in perfect agreement with chRMT. We close with a review of chRMT at nonzero chemical potential. Novel features of spectral universality in nonhermitean matrices will be discussed. As an illustration of mathematical methods used in RMT several important recent results will be derived in all details. We mention the derivation of the microscopic spectral density, the universality proof by Akemann, Damgaard, Magnea and Nishigaki, the spectral density of a chRMT at nonzero temperature and the Stephanov solution for chRMT at nonzero chemical potential.