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Exclusive measurements of the quasi-free $pn \to pp\pi^0\pi^-$ reaction havebeen performed by means of $pd$ collisions at $T_p$ = 1.2 GeV using the WASAdetector setup at COSY. Total and differential cross sections have beenobtained covering the energy region $\sqrt s$ = (2.35 - 2.46) GeV, whichincludes the region of the ABC effect and its associated resonance structure.No ABC effect, {\it i.e.} low-mass enhancement is found in the$\pi^0\pi^-$-invariant mass spectrum -- in agreement with the constraint fromBose statistics that the isovector pion pair can not be in relative s-wave. Atthe upper end of the covered energy region $t$-channel processes for Roper,$\Delta(1600)$ and $\Delta\Delta$ excitations provide a reasonable descriptionof the data, but at low energies the measured cross sections are much largerthan predicted by such processes. Adding a resonance amplitude for theresonance at $m$=~2.37 GeV with $\Gamma$ =~70 MeV and $I(J^P)=~0(3^+)$ observedrecently in $pn \to d\pi^0\pi^0$ and $pn \to d\pi^+\pi^-$ reactions leads to anagreement with the data also at low energies.

Measurement of the $pn \to pp\pi^0\pi^-$ Reaction in Search for the Recently Observed Resonance Structure in $d\pi^0\pi^0$ and $d\pi^+\pi^-$ systems