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Measurement of the $pn \to ppπ^0π^-$ Reaction in Search for the Recently Observed Resonance Structure in $dπ^0π^0$ and $dπ^+π^-$ systems

Exclusive measurements of the quasi-free $pn \to ppπ^0π^-$ 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 $π^0π^-$-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, $Δ(1600)$ and $ΔΔ$ 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 $Γ$ =~70 MeV and $I(J^P)=~0(3^+)$ observed recently in $pn \to dπ^0π^0$ and $pn \to dπ^+π^-$ reactions leads to an agreement with the data also at low energies.