Measurement of the quasi free $np \rightarrow np\pi^{+} \pi^{-}$ and $np \rightarrow pp\pi^{-} \pi^{0}$ reactions at 1.25 GeV With HADES

We present the results of two-pion production in tagged quasi-free np collisions at a deutron incident beam energy of 1.25 GeV/c measured with the High-Acceptance Di-Electron Spectrometer (HADES) installed at GSI. The specific acceptance of HADES allowed for the first time to obtain high-precision data on $\pi^{+} \pi^{-}$ and $\pi^{-} \pi^{0}$ production in np collisions in a region corresponding to large transverse momenta of the secondary particles. The obtained differential cross section data provide strong constraints on the production mechanisms and on the various baryon resonance contributions ($\Delta \Delta$, N(1440), N(1520), $\Delta$ (1600)). The invariant mass and angular distributions from the $np \rightarrow np\pi^{+} \pi{-}$ and $np \rightarrow pp \pi^{-} \pi^{0}$ reactions are compared with different theoretical model predictions.