Isoscalar single pion production in the energy region of Roper and d*(2380) resonances

The isoscalar and isovector parts of the single pion production in NN collisions provide important information about the role of isoscalar and isovector resonance excitations in the course of the reaction process. Whereas the isovector pion production is reasonably well known from threshold up to several GeV, the isoscalar pion production has been determined experimentally so far only for beam energies below 1 GeV. In order to obtain information about the isoscalar strength above 1 GeV, WASA data for the reactions pp → ppπ⁰ and pn → ppπ^- at T_p = 1.2 GeV have been analyzed. Since these reactions were taken in the quasifree mode by use of a deuterium target, the beam energy region of 1.0-1.3 GeV is covered, which is just the region of the Roper N(1440) baryon resonance and of the d*(2380) dibaryon resonance. In the isoscalar Nπ-invariant mass spectrum we observe a pronounced, isolated signal from the Roper resonance. We find no evidence for a decay d*(2380) → NNπ, hence only an upper limit is given.