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

Abstract

In a full QCD lattice study with N f = 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 Z 2- 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 Z 2- noise sources suffice on our sample (about 150 independent QCD configurations at β = 5.6 on 16 3 × 32 with κ sea = 0.157, equivalent to M π / M ϱ = 0.76 (1), to achieve decent signals and adequate fluctuations, rather than 300 such sources as recently used in quenched simulations.