Unexpected neutron/proton ratio and isospin effect in low-energy antiproton induced reactions

Abstract

The inclusive spectra of preequilibrium nucleons produced in low-energy antiproton-nucleus collisions are thoroughly investigated within the framework of the Lanzhou quantum molecular dynamics model for the first time. All possible reaction channels such as elastic scattering, annihilation, charge exchange and inelastic scattering in antibaryon-baryon, baryon-baryon and meson-baryon collisions have been included in the transport model. The unexpected neutron/proton ratio in comparison to the pion and proton induced reactions is caused from the isospin effects of pion-nucleon collisions and the density dependence of symmetry energy. It is found that the $\pi^{-}$-neutron collisions enhance the neutron emission in the antiproton annihilation in a nucleus. Different to heavy-ion collisions, the isospin effects are pronounced at the low kinetic energies. A soft symmetry energy with the stiffness of $\gamma_{s}=$0.5 at subsaturation densities is constrained from the available data of the neutron/proton spectra.