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

The tagged quasi-free $np to nppi^+pi^-$ reaction has been studied experimentally with the High Acceptance Di-Electron Spectrometer (HADES) at GSI at a deuteron incident beam energy of 1.25 GeV/nucleon ($sqrt s sim$ 2.42 GeV/c for the quasi-free collision). For the first time, differential distributions for $pi^{+}pi^{-}$ production in $np$ collisions have been collected in the region corresponding to the large transverse momenta of the secondary particles. The invariant mass and angular distributions for the $nprightarrow nppi^{+}pi^{-}$ reaction are compared with different models. This comparison confirms the dominance of the $t$-channel with $DeltaDelta$ contribution. It also validates the changes previously introduced in the Valencia 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 much better agreement with this model. Our new measurement puts useful constraints for the existence of the conjectured dibaryon resonance at mass M$sim$ 2.38 GeV and with width $Gammasim$ 70 MeV.

Angular distributions of the vector Ay and tensor Ayy, Axx, Axz analyzing powers in the dd->3Hp reaction at 200 MeV

A complete set of analyzing powers for the dd->3Hp reaction at the kinetic beam energy of 200 MeV has been measured in the full angular range in the c.m. frame. The observed signs of the tensor analyzing powers Ayy, Axx, and Axz at forward and backward directions have clearly demonstrated the sensitivity to the ratio of the D- and S-wave components of the triton and deuteron, respectively. The new high-precision data are compared with the prediction of the relativistic multiple-scattering model by using standard wave functions of the three-nucleon bound state and of the deuteron.