The neutron-proton effective mass splitting ($\mathrm{\Delta }{m}_{np}^{*}$) is investigated through analyses of heavy-ion collisions using the improved quantum molecular dynamics 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 $\mathrm{\Delta }{m}_{np}^{*}$, 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 $m_n^{*}>m_p^{*}$ which is consistent with the nucleon-nucleus scattering analysis, while at high kinetic energies, they favor $m_n^{*}<m_p^{*}$. 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.

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• Equations of state of nuclear matter
• Low & intermediate energy heavy-ion reactions
• Symmetry energy