Excitation of the Roper resonance in single- and double-pion production in nucleon-nucleon collisions

The Roper resonance has been a puzzle ever since its discovery in πN phase shifts. Not only its nature has been a matter of permanent debate, also its resonance parameters show a big scatter in their values. In most investigations no apparent resonance signatures could be found in the observables. In these cases the Roper resonance is sensed only very indirectly via complex partial wave analyses. We find direct indications for its excitation in the invariant nπ+ mass spectrum of the pp → npπ+ reaction at M ∼ 1360 MeV with a width of 150 MeV. The values fit very favorably to the most recent phase shift results as well as to the observations at BES. In the near-threshold two-pion production pp → ppπ0π0, where the Roper excitation and its subsequent decays via the routes N* → Δπ→ Nππ and N* → Nσ are the only dominant processes, we find its direct decay into the Nσ channel to be the by far dominating decay process - in favor of a monopole nature of the Roper resonance. Above the Roper region the ΔΔ excitation gets the dominant reactions process for Tp >1 GeV. The situation in this region is discussed, too