Isoscalar Single-Pion Production in the Region of Roper and $d^{\ast }(2380)$ Resonances

Exclusive measurements of the quasi-free $pn\to pp{\pi }^{-}$ and $pp \to pp\pi^0$reactionshavebeenperformedbymeansof$pd$collisionsat$T_p$ = 1.2 GeV using the WASA detector setup at COSY. Total and differential cross sections 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 $\mathrm{\Delta }(1232)$, $N^{\ast }(1440)$ and $d^{\ast }(2380)$ resonance excitations. From these measurements the isoscalar single-pion production has been extracted, for which data existed so far only below $T_p$ = 1 GeV. We observe a substantial increase of this cross section above 1 GeV, which can be related to the Roper resonance $N^{\ast }(1440)$, the strength of which shows up isolated from the $\mathrm{\Delta }$ resonance in the isoscalar $(N\pi {)}_{I=0}$ invariant-mass spectrum. No evidence for a decay of the dibaryon resonance $d^{\ast }(2380)$ into the isoscalar $(NN\pi {)}_{I=0}$ channel is found. An upper limit of 90 $\mu$b (90 $\mathrm{\%}$ C.L.) corresponding to a branching ratio of 5 $\mathrm{\%}$ has been deduced.