Particle Content of the Nonlinear Sigma Model with Theta-Term: a Lattice   Model Investigation
  Using new as well as known results on dimerized quantum spin chains with frustration, we are able to infer some properties on the low-energy spectrum of the O(3) Nonlinear Sigma Model with a topological theta-term. In particular, for sufficiently strong coupling, we find a range of values of theta where a singlet bound state is stable under the triplet continuum. On the basis of these results, we propose a new renormalization group flow diagram for the Nonlinear Sigma Model with theta-term. 
L. Campos Venuti, C. Degli Esposti Boschi, E. Ercolessi, F. Ortolani,   G. Morandi, S. Pasini and M. Roncaglia