SPECTRUM OF THE DIRAC OPERATOR IN A QCD INSTANTON LIQUID - 2 VERSUS 3 COLORS

Approximating the sum over all gauge field configurations in the QCD partition function by a liquid of instantons, we calculate the spectrum of the Dirac operator for two and three colors and for zero, one and two flavors. We find a remarkable difference in the spectrum near zero virtuality between two and three colors, which can be explained in terms of chiral random matrix theory. For two colors the Dirac operator is real, and the appropriate random matrix ensemble has real matrix elements. For three colors the Dirac operator is complex, and the spectrum can be described by a random matrix ensemble with complex matrix elements. These results provide further evidence that the spectrum near zero virtuality is universal and is completely determined by symmetries.