Evaluating sea quark contributions to flavour-singlet operators in lattice QCD

In a full QCD lattice study with Nf = 2 Wilson fermions, we seek to optimize the signals for the disconnected contributions to the matrix element of flavour-singlet operators between nucleon states, which are indicative for sea quark effects. We demonstrate, in form of a fluctuation analysis to the noisy estimator technique, that — in order to achieve a tolerable signal to noise-ratio in full QCD — it is advantageous to work with a Z2-noise source rather than to rely only on gauge invariance to cancel non-gauge-invariant background. In the case of the πN σ-term, we find that 10 Z2-noise sources suffice on our sample (about 150 independent QCD configurations at β = 5.6 on 163 × 32 with κsea = 0.157, equivalent to Mπ/Mϱ = 0.76 (1), to achieve decent signals and adequate fluctuations, rather than 300 such sources as recently used in quenched simulations.