Effective Nucleon Masses from Heavy Ion Collisions

We probe the momentum dependence of the isovector mean-field potential by comparing the energy spectra of neutrons and protons emitted in ${}^{112}$Sn+${}^{112}$Sn and ${}^{124}$Sn+${}^{124}$Sn collisions at incident energies of E/A=50 and 120 MeV. We achieve experimental precision that discriminates between different momentum dependencies for the symmetry mean-field potential. Comparisons of the experimental results to Improved Quantum Molecular Dynamics model calculations with Skyrme Interactions indicate small differences between the neutron and proton effective masses.