A signal of a narrowπNN resonance inpp →ppπ − π +

We report on the first exclusive measurement of the 2π production in NN collisions atT _p=750 MeV using a H₂ cluster jet target and the WASA/PROMICE detector at CELSIUS. In the invariant-mass spectrum $M_{pp\pi ^ - } $ we observe a narrow peak at 2.063(2) GeV with a 4σ deviation relative to the MC simulations for the conventional nonresonant 2π production process. The finding is in agreement with the hypothesis of a narrowπNN resonance withI(J^P)=even (0^?), calledd′, postulated recently to describe the peculiar resonance-like behavior of the pionic double charge exchange on nuclei.     

The (p, π±) reactions on 12,13C at 200 MeV

Angular distributions from the (p, π^±) reactions have been measured at 200 MeV on ¹²C and ¹³C leading to discrete final states in the residual nuclei. A comparison of the cross sections is made between transitions to isobaric analog and non analog final states. Differences between the (p, π⁺) and (p, π^?) reactions are found in the shape and magnitude of the differential cross section as well as in the energy variation.     

Exclusive measurement of the pp → nnπ+π+ reaction at 1.1 GeV

First exclusive data for the $\ensuremath{pp \to nn\pi^+\pi^+}$ reaction have been obtained at CELSIUS with the WASA detector setup at a beam energy of T_p = 1.1 GeV. Total and differential cross-sections disagree with theoretical calculations, which predict the $ \Delta$ $ \Delta$ excitation to be the dominant process at this beam energy. Instead, the data require the excitation of one of the nucleons to a higher-lying $ \Delta$ state, preferably the $\ensuremath{\Delta(1600)P_{33}}$ , to be the leading process.