The $\theta$ vacuum reveals itself as the fundamental theory of the   quantum Hall effect. I. Confronting controversies
  The $\theta$ dependence of the Grassmannian $U(m+n)/U(m)\times U(n)$ non-linear $\sigma$ model is reexamined. This general theory provides an important laboratory for studying the quantum Hall effect, in the special limit $m=n=0$ (replica limit). We discover however that the quantum Hall effect is in fact independent of this limit and exists as a generic topological feature of the theory for all non-negative values of $m$ and $n$. The results are in concflict with many of the historical ideas and expectations on the basis of the large $N$ expansion of the $CP^{N-1}$ or $SU(N)/U(N-1)$ model 
A.M.M. Pruisken, M.A. Baranov and M. Voropaev