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

Abstract

Exclusive measurements of the quasi-free $pn\to pp\ddot{I}{€}^{-}$ and $pp\to pp\ddot{I}{€}^0$ 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 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 $\hat{I}”(1232)$, $N^{\ast }(1440)$ 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 $\hat{I}”$ resonance in the isoscalar $(N\ddot{I}€{)}_{I=0}$ invariant-mass spectrum. No evidence for a decay of the dibaryon resonance $d^{\ast }(2380)$ into the isoscalar $(NN\ddot{I}€{)}_{I=0}$ channel is found. An upper limit of 90 $\hat{I}¼$b (90 $\%$ C.L.) corresponding to a branching ratio of 5 $\%$ has been deduced.

erratum: the upper limit for the branching ratio of the decay $d^{\ast }(2380)\to (NN\ddot{I}€{)}_{I=0}$ is a factor of two smaller than given in the previous version of this article