A partially quenched analysis of the eta-eta' system in N_f=2 QCD
  We report on a first, comprehensive partially quenched study of the eta-eta' problem, based on SESAM configurations on a 16^3x32 lattice at beta=5.6 QCD with two (mass degenerate) active sea quark flavours. By means of the spectral approximation of the two-loop (hairpin) diagrams, we find clear plateau formation in the effective masses which enables us both to determine the eta-eta' mass matrix and the alpha-parameter in the effective chiral Lagrangian for the flavour singlet sector, alpha=0.028 +- 0.013. 
H. Neff, Th. Lippert, J. Negele and K. Schilling