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

TL;DR

This study measures isoscalar single-pion production in proton-neutron and proton-proton collisions at energies covering key resonances, revealing a rise above 1 GeV linked to the Roper resonance and no evidence for decay of the $d^*(2380)$ dibaryon.

Contribution

First measurements of isoscalar single-pion production above 1 GeV, isolating the Roper resonance contribution and setting limits on $d^*(2380)$ decay channels.

Findings

Significant increase in isoscalar cross section above 1 GeV.

Isolation of Roper resonance in the invariant-mass spectrum.

No evidence for $d^*(2380)$ decay into isoscalar $(NN o ext{pion})$ channel.

Abstract

Exclusive measurements of the quasi-free $pn \to pp\pi^-$ and $pp \to pp\pi^0$ 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 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 $\Delta(1232)$, $N^*(1440)$ and $d^*(2380)$ 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 $N^*(1440)$, the strength of which shows up isolated from the $\Delta$ resonance in the isoscalar $(N\pi)_{I=0}$ invariant-mass spectrum. No evidence for a decay of the dibaryon resonance $d^*(2380)$ into the isoscalar $(NN\pi)_{I=0}$ channel is found. An upper limit of 90 $\mu$b (90 $\%$ C.L.) corresponding to a branching ratio of 5 $\%$ has been deduced.