Fast nucleon emission as a probe of the momentum dependence of the symmetry potential

We investigate the structure of the nonlocal contributions to the potential part of the symmetry term of the nuclear equation of state (EOS). Such momentum dependence of the symmetry potential leads to a splitting of the effective masses of protons and neutrons in asymmetric matter that will directly affect the nucleus-nucleus reaction dynamics. Based on microscopic transport simulations we suggest some rather sensitive observables in collisions of neutron-rich (unstable) ions at intermediate energies. In particular we focus the attention on preequilibrium nucleon emissions. We discuss interesting correlations between the $N/Z$ content of the fast emitted particles and their rapidity or transverse momentum, that show a nice dependence on the prescription used for the effective mass splitting.

Figure 1

(Color online) Energy dependence of the Lane potential for the used parametrizations. (Inset) The effective mass splitting, related to the slope of the Lane potential (for the asymmetry $I=0.2$).

Figure 2

(Color online) Gas asymmetry as a function of time in a central collision ${}^{132}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ at 50 A MeV for two opposite choices of mass splitting. (Left panel) asy-soft EOS; (right panel) asy-stiff EOS.

Figure 3

(Color online) Transverse momentum dependence of the neutron/proton ratio in the rapidity range ${}^{132}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ for a central reaction ${}^{132}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ at 50 A MeV. Comparison between two choices of symmetry energy stiffness at the two different times defined in the text.

Figure 4

(Color online) Same as described in the legend to Fig. 3, for two opposite choices of mass splitting.

Figure 5

(Color online) Same as Fig. 4, but for an incident energy of 100 A MeV [note the reduced pre-equilibrium (left panel) and freeze-out (right panel) times].

Figure 6

(Color online) The asymmetry of fast-emitted particles as a function of rapidity, for the reaction ${}^{132}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ at 100 MeV/A, $b=2$ fm.