Finite density 2d O(3) sigma model: dualization and numerical simulations

The action of the 2d O(3) non-linear sigma model on the lattice in a bath of particles, when expressed in terms of standard O(3) degrees of freedom, is complex. A reformulation of the model in terms of new variables that makes the action real is presented. This reshaping enables us to utilize Monte Carlo simulations based on usual importance sampling. Several observables, including the correlation function and the mass gap, are measured.