Study of the Reaction $np \rightarrow np \pi^+ \pi^-$ at Intermediate   Energies
  The reaction $np \rightarrow np \pi^+ \pi^-$ was studied at the various momenta of incident neutrons. It was shown that the characteristics of the reaction at the momenta above 3 GeV/c could be described by the model of reggeized $\pi$ exchange (OPER). At the momenta below 3 GeV/c, it was necessary to use additionally the mechanism of one baryon exchange (OBE). 
A.P. Jerusalimov, Yu.A.Troyan, A.Yu.Troyan, A.V.Belyaev, E.B.Plekhanov
hep-ex hep-ph