Experimental study of the p d ( d p ) → He 3 π π reactions close to threshold

New experimental data on the $pd\ensuremath{\rightarrow}^{3}\mathrm{He}\phantom{\rule{0.16em}{0ex}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ reaction obtained with the COSY-MOMO detector below the three-pion threshold are presented. The reaction was also studied in inverse kinematics with a deuteron beam and the higher counting rates achieved were especially important at low excess energies. The comparison of these data with inclusive $pd\ensuremath{\rightarrow}^{3}\mathrm{He}\phantom{\rule{0.16em}{0ex}}{X}^{0}$ rates allowed estimates also to be made of ${\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ production. The results confirm our earlier findings that, close to threshold, there is no enhancement at low excitation energies in the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ system, where the data seem largely suppressed compared with phase space. Possible explanations for this behavior, such as strong $p$ waves in the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ system or the influence of two-step processes, are explored.