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

The inclusive spectra of pre-equilibrium nucleons produced in low-energy antiproton-nucleus collisions are thoroughly investigated within the the Lanzhou quantum molecular dynamics transport approach for the first time. The reaction channels of elastic scattering, annihilation, charge exchange, and inelastic processes in antibaryon-baryon, baryon-baryon, and meson-baryon collisions have been implemented in the model. The unexpected neutron to proton yield ratios are caused from the isospin effects of pion-nucleon collisions and the symmetry energy. It is found that the ${\pi }^{-}$-neutron collisions enhance the neutron emission in the antiproton annihilation in a nucleus. 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.

Figure 1

Kinetic energy spectra of neutrons and protons produced in antiproton annihilations on carbon, copper, and uranium at an incident momentum of 200 MeV/$c$ and compared with the available data at the LEAR facility [27].

Figure 2

Impacts of $\pi \mathrm{N}$ absorption and symmetry energy on the yield ratios of neutrons to protons in the $\overline{p}+{}^{65}\mathrm{Cu}$ reaction at 200 MeV/$c$ and the experimental data with the stopped $\overline{p}$ on ${}^{\mathrm{nat}}\mathrm{Cu}$ [27].

Figure 3

Nuclear density profiles at which antiprotons are annihilated on the targets of ${}^{65}\mathrm{Cu}, {}^{197}\mathrm{Au}$, and ${}^{238}\mathrm{U}$.

Figure 4

The $n/p$ ratios in antiproton-induced reactions on ${}^{12}\mathrm{C}, {}^{\mathrm{nat}}\mathrm{Cu}, {}^{197}\mathrm{Au}$, and ${}^{238}\mathrm{U}$ with different stiffnesses of symmetry energies and compared with the LEAR data [27].