Multicanonical study of D=2 O(3) nonlinear σ-model

We present a new and exploratory approach to determine the $\Delta\beta(\beta)$-shift in the $O(3)$ nonlinear $\sigma$-model. The method is based on a scaling hypothesis for a free energy difference, which is assumed to be valid in a situation where the mass gap correlation length $\xi$ is of the order or larger than the linear extent $L$ of the considered square lattice sizes. The free energy difference arises from the finite volume constraint effective potential of the theory. While the constraint effective potential is calculated in numerical simulations employing a variant of the multicanonical ensemble on medium sized lattices, it is possible to estimate $\Delta\beta(\beta)$ up to a value of $\beta=2.8$ in the standard parameterization of the model.