Beam energy dependence of the relativistic retardation effects of electrical fields on the π⁻/π⁺ ratio in heavy-ion collisions

In this article we investigate the beam energy dependence of relativistic retardation effects of electrical fields on the single and double $\pi^{-}/\pi^{+}$ ratios in three heavy-ion reactions with an isospin- and momentum-dependent transport model IBUU11. With the beam energy increasing from 200 to 400 MeV/nucleon, effects of the relativistically retarded electrical fields on the $\pi^{-}/\pi^{+}$ ratio are found to increase gradually from negligibly to considerably significant as expectedly; it is however, the interesting observation is the relativistic retardation effects of electrical fields on the $\pi^{-}/\pi^{+}$ ratio are becoming gradually insignificant as the beam energy further increasing from 400 to 800 MeV/nucleon. Moreover, we also investigate the isospin dependence of relativistic retardation effects of electrical fields on the $\pi^{-}/\pi^{+}$ ratio in two isobar reaction systems of $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr at the beam energies from 200 to 800 MeV/nucleon. It is shown that the relativistic retardation effects of electrical fields on the $\pi^{-}/\pi^{+}$ ratio are independent of the isospin of reaction. Furthermore, we also examine the double $\pi^{-}/\pi^{+}$ ratio in reactions of $^{96}$Zr+$^{96}$Zr over $^{96}$Ru+$^{96}$Ru at the beam energies from 200 to 800 MeV/nucleon with the static field and retarded field, respectively. It is shown the double $\pi^{-}/\pi^{+}$ ratio from two reactions is still an effective observable of symmetry energy without the interference of electrical field due to using the relativistic calculation compared to the nonrelativistic calculation.