Low fermionic eigenmode dominance in QCD on the lattice - art. no. 114509

We demonstrate the utility of a spectral approximation to fermion loop oper ators using low-lying eigenmodes of the Hermitian Dirac-Wilson matrix, Q = gamma M-5. The investigation is based on a total of 400 full QCD vacuum con figurations, with two degenerate flavors of dynamical Wilson fermions at be ta = 5.6, at two different sea quark masses. The spectral approach is highl y competitive for accessing both topological charge and disconnected diagra ms, on large lattices and small quark masses. We propose suitable partial s ummation techniques that provide sufficient saturation for estimating Tr Q( -1), which is related to the topological charge. In the effective mass plot of the eta' meson we achieved a consistent early plateau formation, by gro und state projecting the connected piece of its propagator.