Operator content of the fusion cyclic solid-on-solid model

The fusion model of the L-state cyclic solid-on-solid (CSOS) model is considered at criticality. Each Z_L-charge sector of the fusion CSOS model is related to a sum of total spin sectors of the fusion vertex model with a seam on which the vertex weights are modified by a phase factor. The latter in turn becomes the higher-spin XXZ quantum chain with a twisted boundary condition in the extremely anisotropic limit. Using the known operator content of the higher-spin XXZ chain, the authors deduce that of the fusion CSOS model. The modular invariant partition function of the fusion CSOS at fusion level k is a sum of products of the Z(k) parafermionic and free boson sectors with the effective coupling g'=L²(k-lambda/pi) for L odd and g"=L ²(1/k-lambda/pi)/4 for L even, where lambda is the crossing parameter. Where L is odd and a multiple of k, or when L/2 is a multiple of k for L even, the modular invariant partition function becomes a simple product of the Z(k) parafermionic and the Gaussian partition function.