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

TL;DR

This study measures the $pn o pp\pi^0\pi^-$ reaction near 2.35-2.46 GeV to investigate resonance structures and the ABC effect, finding no low-mass enhancement and suggesting a resonance at 2.37 GeV explains the data.

Contribution

First measurement of the $pn o pp\pi^0\pi^-$ reaction in this energy range, revealing the role of a 2.37 GeV resonance in explaining cross sections and resonance structures.

Findings

No ABC effect observed in the $\pi^0\pi^- ext{invariant mass spectrum}

At high energies, t-channel processes describe the data well

At low energies, a resonance at 2.37 GeV improves data agreement

Abstract

Exclusive measurements of the quasi-free $pn \to pp\pi^0\pi^-$ 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 $\pi^0\pi^-$-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, $\Delta(1600)$ and $\Delta\Delta$ 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 $\Gamma$ =~70 MeV and $I(J^P)=~0(3^+)$ observed recently in $pn \to d\pi^0\pi^0$ and $pn \to d\pi^+\pi^-$ reactions leads to an agreement with the data also at low energies.