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Exclusive measurements of the quasi-free $pn \to pp\pi^-$ and $pp \topp\pi^0$ reactions have been performed by means of $pd$ collisions at $T_p$ =1.2 GeV using the WASA detector setup at COSY. Total and differential crosssections have been obtained covering the energy region $T_p = 0.95 - 1.3$ GeV($\sqrt s$ = 2.3 - 2.46 GeV), which includes the regions of $\Delta(1232)$,$N^*(1440)$ and $d^*(2380)$ resonance excitations. From these measurements theisoscalar single-pion production has been extracted, for which data existed sofar only below $T_p$ = 1 GeV. We observe a substantial increase of this crosssection above 1 GeV, which can be related to the Roper resonance $N^*(1440)$,the strength of which shows up isolated from the $\Delta$ resonance in theisoscalar $(N\pi)_{I=0}$ invariant-mass spectrum. No evidence for a decay ofthe dibaryon resonance $d^*(2380)$ into the isoscalar $(NN\pi)_{I=0}$ channelis found. An upper limit of 90 $\mu$b (90 $\%$ C.L.) corresponding to abranching ratio of 5 $\%$ has been deduced.

Isoscalar Single-Pion Production in the Region of Roper and $d^*(2380)$ Resonances