Measurement of the $pn\to pp\ddot{I}{€}^0\ddot{I}{€}^{-}$ Reaction in Search for the Recently Observed Resonance Structure in $d\ddot{I}{€}^0\ddot{I}{€}^0$ and $d\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ systems

Abstract

Exclusive measurements of the quasi-free $pn\to pp\ddot{I}{€}^0\ddot{I}{€}^{-}$ reaction 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 $\sqrt{s}$ = (2.35 - 2.46) GeV, which includes the region of the ABC effect and its associated resonance structure. No ABC effect, {\it i.e.} low-mass enhancement is found in the $\ddot{I}{€}^0\ddot{I}{€}^{-}$-invariant mass spectrum -- in agreement with the constraint from Bose statistics that the isovector pion pair can not be in relative s-wave. At the upper end of the covered energy region $t$-channel processes for Roper, $\hat{I}”(1600)$ and $\hat{I}”\hat{I}”$ excitations provide a reasonable description of the data, but at low energies the measured cross sections are much larger than predicted by such processes. Adding a resonance amplitude for the resonance at $m$=~2.37 GeV with $\hat{I}“$ =~70 MeV and $I(J^P)=0(3^{+})$ observed recently in $pn\to d\ddot{I}{€}^0\ddot{I}{€}^0$ and $pn\to d\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ reactions leads to an agreement with the data also at low energies.