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 η − η′ problem, based on SESAM configuration on a 163 × 32 lattice at β = 5.6 QCD with two (mass degenerate) active sea quark flavours, Nf = 2. 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 η − η′ mass matrix and the α-parameter in the effective chiral Lagrangian for the flavour singlet sector, α = 0.028 ± 0.013.