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

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 $d^{\ast }(2380)$). 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.

Comments: 16 pages, 9 figures; version accepted for publication in Phys. Rev. D