3D loop models and the CP^{n-1} sigma model
  Many statistical mechanics problems can be framed in terms of random curves; we consider a class of three-dimensional loop models that are prototypes for such ensembles. The models show transitions between phases with infinite loops and short-loop phases. We map them to $CP^{n-1}$ sigma models, where $n$ is the loop fugacity. Using Monte Carlo simulations, we find continuous transitions for $n=1,2,3$, and first order transitions for $n\geq 5$. The results are relevant to line defects in random media, as well as to Anderson localization and $(2+1)$-dimensional quantum magnets. 
Adam Nahum, J. T. Chalker, P. Serna, M. Ortu\~no and A. M. Somoza
cond-mat.stat-mech cond-mat.dis-nn