Quantum mechanically induced Wess-Zumino term in the principal chiral model

It is argued that in the two-dimensional principal chiral model, the Wess-Z umino term may be induced quantum mechanically in a manner analogous to the theta-term in QCD. Our argument is based on the fact that the configuratio n space of the model has a structure topologically identified as a sphere, and that the Dirac monopole potential, which is induced on the sphere by in equivalent quantizations, appears in the guise of the Wess-Zumino term. Our result suggests that, at the critical value lambda(2) = 8 pi/\k\ of the co upling constant, the principal chiral model belongs to the same family of m odels with the Wess-Zumino-Novikov-Witten model at the quantum level.