HADES

We present the results of two-pion production in tagged quasi-free $np$ collisions at a proton incident beam energy of 1.25 GeV 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 $np \to np\pi^{+}\pi^{-}$ and $np \to pp\pi^{-}\pi^{0}$ reactions are compared with different theoretical model predictions.