This model is solvable using the (coordinate) Bethe ansatz. The scattering phase shift function is ϕ ϕ ( k a ( λ λ a ) , k b ( λ λ b ) ) = θ θ 2 ( λ λ a − − λ λ b ) , with θ θ n ( λ λ ) ≡ ≡ 2 arctan ⁡ ⁡ 2 λ λ n in which the momentum has been conveniently reparametrized as k ( λ λ ) = π π − − 2 arctan ⁡ ⁡ 2 λ λ in terms of the rapidity λ λ . The (here, periodic) boundary conditions impose the Bethe equations