More methods for calculating the topological charge (density) of SU(N) lattice gauge fields in 3+1 dimensions

Abstract: We revisit old ideas that smearing or blocking an SU(N) lattice gauge field, or averaging over an ensemble of fields created in the neighbourhood of that field, can reduce the high frequency fluctuations sufficiently that the naive lattice operator for the topological charge density is able to provide a reliable measure of the topological charge of the field. We show that these three methods do indeed provide additional simple methods for calculating the total topological charge, with smearing particularly economical at current couplings. More interestingly, the ensemble average method can also be used to expose the distribution in space-time of the topological charge and this conceptually transparent, albeit computationally expensive, method provides a useful benchmark against which to compare other methods. Using this benchmark we find that a few smearing steps are also reliable in exposing the distribution in space-time of the topological charge, thus providing a very economical and simple method for doing so. We also use the same benchmark to determine what is the number of `cooling' sweeps one needs to perform in order to expose the charge density reliably.