Investigation of the neutron-proton effective mass splitting via heavy ion collisions: Constraints and Implications

Abstract

The neutron-proton effective mass splitting ($\hat{I}”m_{np}^{\ast }$) is investigated through analyses of heavy-ion collisions using the improved quantum molecular dynamics (ImQMD) model with both standard and extended Skyrme interactions. By uncovering the strong correlation between the slope of the neutron-to-proton yield ratio with respect to the kinetic energy (i.e., $S_{n/p}$) and $\hat{I}”m_{np}^{\ast }$, we reveal that the constraints of the neutron-proton effective mass splitting via heavy ion collisions depend on the kinetic energy region of the emitted nucleons. At low kinetic energies, the data favor $m_n^{\ast }>m_p^{\ast }$ which is consistent with the nucleon-nucleus scattering analysis, while at high kinetic energies, they favor $m_n^{\ast }<m_p^{\ast }$. Our findings partly resolve the longstanding discrepancy in the constraints of neutron-proton effective mass splitting with heavy ion collisions and nucleon-nucleus scattering, and significantly advance the understanding of nucleon effective mass splitting through heavy ion collisions.

7 pages, 5 figures