  The Operator Product Expansion approach to scattering amplitudes in maximally
supersymmetric gauge theory operates in terms of pentagon transitions for
excitations propagating on a color flux tube. These obey a set of axioms which
allow one to determine them to all orders in 't Hooft coupling and confront
against explicit calculations. One of the simplifying features of the formalism
is the factorizability of multiparticle transitions in terms of single-particle
ones. In this paper we extend an earlier consideration of a sector populated by
one kind of excitations to the case of a system with fermionic as well as
bosonic degrees of freedom to address the origin of the factorization. While
the purely bosonic case was analyzed within an integrable noncompact open-spin
chain model, the current case is solved in the framework of a supersymmetric
sl(2|1) magnet. We find the eigenfunctions for the multiparticle system making
use of the R-matrix approach. Constructing resulting pentagon transitions, we
prove their factorized form. The discussion corresponds to leading order of
perturbation theory.
