Topological susceptibility on the lattice: The two-dimensional O(3) sigma model

As a test of the procedures used in lattice gauge theories, we study the topological susceptibility χ in a two-dimensional O(3) σ or CP1 model. We determine χ by defining the density of topological charge as a local operator on the lattice. Following the prescriptions of field theory we perform the additive and multiplicative renormalizations needed to extract χ from Monte Carlo data. We also determine χ by the cooling method, finding consistent results. A combined use of cooling and field theory again gives the same result and insight into the renormalization mechanism. Finally we give a direct determination, by Monte Carlo techniques, of both the multiplicative and additive renormalizations, by heating configurations with a definite number of instantons. The results are consistent with perturbation theory.