Isospin symmetry breaking in double-pion production in the region of ${d^*(2380)}$ and the scalar ${\sigma}$ meson

TL;DR

This paper provides a theoretical interpretation of experimental data on double-pion production in proton-neutron collisions, explaining isospin symmetry breaking effects via a scalar sigma meson decay from a dibaryon resonance.

Contribution

It introduces a model based on the decay of a dibaryon resonance involving a sigma meson to explain isospin symmetry breaking in double-pion production.

Findings

The model explains the ABC effect variation between different reactions.

Isospin symmetry violation is attributed to sigma meson decay.

Partial chiral symmetry restoration influences the decay process.

Abstract

The first attempt is made to provide a quantitative theoretical interpretation of the WASA-at-COSY experimental data on the basic double-pion production reactions $pn \to d \pi^0\pi^0$ and $pn \to d \pi^+\pi^-$ in the energy region $T_p =1$ - $1.3$ GeV [P. Adlarson et al., Phys. Lett. B 721, 229 (2013)]. The data are analyzed within a model based on production and decay of an intermediate $I(J^P)=0(3^+)$ dibaryon resonance $\mathcal{D}_{03}$ (denoted also as $d^*(2380)$). The observed decrease of the near-threshold enhancement (the so-called ABC effect) in the reaction $pn \to d \pi^+\pi^-$ in comparison to that in the reaction $pn \to d \pi^0\pi^0$ is explained (at least partially) to be due to isospin symmetry violation in the two-pion decay of an intermediate near-threshold scalar $\sigma$ meson emitted from the $\mathcal{D}_{03}$ dibaryon resonance under conditions of the partial chiral symmetry restoration.