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The tagged quasi-free $np \to np\pi^+\pi^-$ reaction has been studiedexperimentally with the High Acceptance Di-Electron Spectrometer (HADES) at GSIat a deuteron incident beam energy of 1.25 GeV/nucleon ($\sqrt s \sim$ 2.42GeV/c for the quasi-free collision). For the first time, differentialdistributions for $\pi^{+}\pi^{-}$ production in $np$ collisions have beencollected in the region corresponding to the large transverse momenta of thesecondary particles. The invariant mass and angular distributions for the$np\rightarrow np\pi^{+}\pi^{-}$ reaction are compared with different models.This comparison confirms the dominance of the $t$-channel with $\Delta\Delta$contribution. It also validates the changes previously introduced in theValencia model to describe two-pion production data in other isospin channels,although some deviations are observed, especially for the $\pi^{+}\pi^{-}$invariant mass spectrum. The extracted total cross section is also in muchbetter agreement with this model. Our new measurement puts useful constraintsfor the existence of the conjectured dibaryon resonance at mass M$\sim$ 2.38GeV and with width $\Gamma\sim$ 70 MeV.

Study of the quasi-free $np \to np \pi^+\pi^-$ reaction with a deuterium beam at 1.25 GeV/nucleon