Flavor singlet pseudoscalar masses in N f =2 QCD

We perform a lattice mass analysis in the flavor singlet pseudoscalar channel on the SESAM and TχL full QCD vacuum configurations, with 2 active flavors of dynamical Wilson fermions at β=5.6. At our inverse lattice spacing, a -1 ≈2.3 GeV, we retrieve by a chiral extrapolation to the physical light quark masses the value m η ′=3.7 -4 +8 m π . A crude extrapolation from (Nf=3) phenomenology would suggest m η ′≈5.1m π for Nf=2 QCD. We verify that the mass gap between the singlet state η′ and the π flavor triplet state is due to gauge configurations with nontrivial topology.