Probing Neutron-Proton Effective Mass Splitting Using Heavy Ion Collisions

Single-nucleon potential is an important input to the transport model.Both the isovector part（symmetry potential）and the isoscalar part（scalar potential）of the single-particle potential depend on momentum due to the non-locality of the nucleon-nucleon interactions and the Pauli exchange effects in many-fermion systems.However,in trans-port models of heavy ion collisions,the momentum dependence of the single-nucleon potential has not been considered in a more detailed way until the last two or three decades.Moreover,it has been found that the momentum dependence of the single-nucleon potential affects many experimentally sensitive observables of symmetry en-ergy.The momentum dependence of the symmetry potential can be characterized by nucleon effective mass.For asymmetric nuclear matter,there is generally a difference between the effective masses of neutrons and protons,and this difference can be de-scribed by the neutron-proton effective mass splitting m_n-p^*=(m_n^*-₉)-m_p^*).The neutron-proton effective mass splitting affects the neutron/proton ratio during the early evolution of the universe,in-medium nucleon-nucleon scattering cross section,and it is a fun-damental quantity for understanding many phenomena of nuclear physics and nuclear astrophysics.In this work,the nucleonic mean-field potential,nucleon effective mass,and ini-tial density distributions of projectile and target nucleus are obtained from the Skyrme nucleon-nucleon effective interactions based previous works,which are self-consistently used in the Boltzmann-Uehling-Uhlenbeck（BUU）transport model.Six sets of Skyrme parameters（Sk I2,Gs,KDE0v1,NRAPR,BSk9,and SV-mas08）corresponding to three symmetry energy regions with different neutron-proton effective mass splittings are used in the BUU transport model to simulate the¹²⁴Sn+¹²⁴Sn and¹¹²Sn+¹¹²Sn colli-sions.It is found that the free neutron/proton double ratio caused by the neutron-proton effective mass splitting effect is more pronounced in higher kinetic energy of nucleons.In addition,the comparison with NSCL experimental data indicates that among the six sets of Skyrme interactions,the double neutron/proton ratio results corresponding to the KDE0v1 interaction are more consistent with the experimental data.