  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.
