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

TL;DR

This study uses heavy-ion collision data and advanced modeling to constrain the neutron-proton effective mass splitting, revealing energy-dependent preferences that help resolve previous discrepancies and deepen understanding of nucleon effective masses.

Contribution

It provides new constraints on neutron-proton effective mass splitting by analyzing energy-dependent data from heavy-ion collisions with improved models.

Findings

At low energies, data favor $m_n^*>m_p^*$ consistent with scattering analysis.

At high energies, data favor $m_n^*<m_p^*$, indicating energy dependence.

Partly resolves longstanding discrepancies in effective mass splitting constraints.

Abstract

The neutron-proton effective mass splitting ($\Delta m^*_{np}$) 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 $\Delta m^*_{np}$, 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.