On Lorentz invariance and supersymmetry of four particle scattering   amplitudes in $S^N\R^8$ orbifold sigma model
  The $S^N\R^8$ supersymmetric orbifold sigma model is expected to describe the IR limit of the Matrix string theory. In the framework of the model the type IIA string interaction is governed by a vertex which was recently proposed by R.Dijkgraaf, E.Verlinde and H.Verlinde. By using this interaction vertex we derive all four particle scattering amplitudes directly from the orbifold model in the large $N$ limit. 
G.Arutyunov, S.Frolov and A.Polishchuk