  We investigate the critical behaviour at theta=pi of the two-dimensional O(3)
nonlinear sigma model with topological term on the lattice. Our method is based
on numerical simulations at imaginary values of theta, and on scaling
transformations that allow a controlled analytic continuation to real values of
theta. Our results are compatible with a second order phase transition, with
the critical exponent of the SU(2)_1 Wess-Zumino-Novikov-Witten model, for
sufficiently small values of the coupling.

High Energy Physics - Lattice