S-matrix of the Yang-Lee edge singularity in two dimensions

The deformation by a relevant operator of the simplest, albeit nonunitary, conformal field theory, that of the Yang-Lee edge singularity, is analysed. It is shown that additional conserved currents exist which imply the factorization of the S-matrix, whose form is explicitly derived. The spectrum contains a single particle. It is proved that any such theory with only one particle must necessarily be nonunitary.