We study the question of diagonalizability of the Hamiltonian for the Faddeev-Reshetikhin (FR) model in the two particle sector. Although the two particle S-matrix element for the FR model, which may be relevant for the quantization of strings on AdS 5 ×S 5 , has been calculated recently using field theoretic methods, we find that the Hamiltonian for the system in this sector is not diagonalizable. We trace the difficulty to the fact that the interaction term in the Hamiltonian violating Lorentz invariance leads to discontinuity conditions (matching conditions) that cannot be satisfied. We determine the most general quartic interaction Hamiltonian that can be diagonalized. This includes the bosonic Thirring model as well as the bosonic chiral Gross-Neveu model which we find share the same S-matrix. We explain this by showing, through a Fierz transformation, that these two models are in fact equivalent. In addition, we find a general quartic interaction Hamiltonian, violating Lorentz invariance, that can be diagonalized with the same two particle S-matrix element as calculated by Klose and Zarembo for the FR model. This family of generalized interaction Hamiltonians is not Hermitian, but is P T symmetric. We show that the wave functions for this system are also P T symmetric. Thus, the theory is in a P T unbroken phase which guarantees the reality of the energy spectrum as well as the unitarity of the S-matrix. simpler (FR) model proposed by Faddeev and Reshetikhin is quite important from this point of view in connection with quantization of strings on AdS 5 × S 5 [2,28,31,[33][34][35][36][37][38][39][40].Recently, Klose and Zarembo (KZ) [41] calculated the S-matrix for a number of 1 + 1 dimensional integrable models (see also [42][43][44][45][46]) using standard field theoretic methods [47][48][49][50] in a simple manner. The simplicity of their method arises from the fact that the calculations are carried out in the wrong vacuum [51,52]. In this case, it is well known, for example, that in the two particle sector, the contribution to the S-matrix comes only from the bubble diagrams and if the system is integrable, all other scattering elements can be related to the two particle S-matrix [53][54][55][56]. One of the models studied by KZ is indeed the FR model, whose S-matrix element for the positive energy two particle states has a simple form that reflects the violation of Lorentz invariance present in the interaction Hamiltonian. This is interesting and, in fact, is relevant as a first step in understanding the quantization of the string itself. However, since the S-matrix element for the FR model is calculated in the wrong vacuum, it is necessary, as a next step, to go to the true vacuum to extract physical results [57]. This can be carried out by diagonalizing the (quartic) Hamiltonian of the theory in this sector.In this paper, we study the question of diagonalization of the two particle Hamiltonian for the FR model systematically. Surprisingly, we find that the quartic Hamiltonian for the FR model cannot be...