Probing the QCD vacuum with flavour singlet objects: $\eta'$ on the   lattice
  We present a study on the direct determination of the $\eta '$ mass on the full set of SESAM and T$\chi$L QCD vacuum configurations with 2 active flavours of Wilson fermions, at $\beta = 5.6$. We observe a definite dependency of the two-loop correlator on the topological charge sector. 
T. Struckmann, K. Schilling, P. Ueberholz, N.Eicker, S. Guesken, Th.   Lippert, H. Neff, B. Orth, J. Viehoff