Low-Lying Fermion Modes: Dynamical versus Quenched

The eigenmodes of the Dirac operator carry information about the topological content of the background gauge field. The Atiyah-Singer index theorem tells us that in a sector with topological charge Qt the Dirac operator has at least |Qt| zero modes. In the presence of light dynamical fermions, the light quark determinant should suppress topological sectors with large |Qt|. As a consequence we expect a reduced value for the topological susceptibility