Analysis of the Reaction: $np \rightarrow np \pi^+ \pi^-$ from the Point   of View of Oper-Model
  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
nucl-th hep-ph