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Probing the momentum-dependence of the symmetry potential by the free n/p ratio of preequilibrium emission

Based on an isospin and momentum-dependent transport model, we studied the effect of momentum-dependent symmetry potential on the free neutron to proton ratio of preequilibrium nucleon emission. It is found that for $^{132}Sn+^{124}Sn$ reaction at 400 MeV/nucleon incident beam energy, free n/p ratio of preequilibrium nucleon emission mainly probe the momentum-dependence of symmetry potential at nucleon momentum around 400 $\sim$ 600 MeV/c. Whereas for 200 MeV/nucleon incident beam energy, this observable mainly probe the momentum-dependence of symmetry potential at nucleon momentum around 200 $\sim$ 400 MeV/c. To probe the symmetry energy/potential using free n/p ratio, not all the details of the momentum-dependence of the symmetry potential are important, the values of symmetry potential at only certain momentum range are crucial for an observable. It is important to input reasonable density- and momentum-dependence of the symmetry potential according to the magnitude of incident beam energy of heavy-ion collisions. The present experimental data on the symmetry potential are not enough for probing the density-dependent symmetry energy. More experimental data (such as nucleon and nuclei's scattering experiments at different nucleonic momenta and densities) on the symmetry potential are therefore needed to pin down the density-dependent symmetry energy.