AN IMPROVED ESTIMATOR FOR THE CORRELATION-FUNCTION OF 2D NONLINEAR SIGMA-MODELS

I present a new improved estimator for the correlation function of 2D nonlinear sigma models. Numerical tests for the 2D XY model and the 2D O(3)-invariant vector model were performed, For small physical volume a reduction of the statistical error of the finite system correlation length by a factor of up to 30 compared to the cluster-improved estim ator was observed. This improvement allows for a very accurate determi nation of the running coupling proposed by M. Luscher et al. for 2D O( N)-invariant vector models.