Effective Nucleon Masses in Symmetric and Asymmetric Nuclear Matter

The momentum and isospin dependence of the in-medium nucleon mass are studied. Two definitions of the effective mass, i.e. the Dirac mass $m^*_D$ and the nonrelativistic mass $m^*_{NR}$ which parameterizes the energy spectrum, are compared. Both masses are determined from relativistic Dirac-Brueckner-Hartree-Fock calculations. The nonrelativistic mass shows a distinct peak around the Fermi momentum. The proton-neutron mass splitting in isospin asymmetric matter is $m^*_{D,n} <m^*_{D,p}$ and opposite for the nonrelativistic mass, i.e. $m^*_{NR,n} >m^*_{NR,p}$, which is consistent with nonrelativistic approaches.