Flavor singlet pseudoscalar masses in $N_f=2\mathrm{}$ QCD

We perform a lattice mass analysis in the flavor singlet pseudoscalar channel on the SESAM and $\mathrm{T}\chi \mathrm{L}$ full QCD vacuum configurations, with 2 active flavors of dynamical Wilson fermions at $\beta =\mathrm{5.6.}\mathrm{}$ At our inverse lattice spacing, $a^{-1}\approx 2.3$ GeV, we retrieve by a chiral extrapolation to the physical light quark masses the value $m_{{\eta }^{\prime }}{=3.7\mathrm{}}_{-4}^{+8\mathrm{}}{m}_{\pi }.$ A crude extrapolation from ${(N}_f=3)$ phenomenology would suggest $m_{{\eta }^{\prime }}\approx {5.1m}_{\pi }$ for $N_f=2\mathrm{}$ QCD. We verify that the mass gap between the singlet state ${\eta }^{\prime }$ and the $\pi$ flavor triplet state is due to gauge configurations with nontrivial topology.