Neutron-proton Effective Mass Splitting And Nuclear Symmetry Energy

Nuclear symmetry energy and neutron-proton effective mass splitting has important implications for nuclear physics and astronomy.Symmetry energy at sub-saturation densities and supra-normal densities has no consistent conclusion and is poorly known especially at supra-normal densities.Currently there is no consensus as to whether the neutron-proton effective mass splitting mn-p*=(mn*-mp*)/m is positive,negative,or zero.So the density dependence of nuclear symmetry energy Esym(?)and the neutron-proton effective mass splitting mn-p*become a hot topic recently.Based on an improved isospin-and momentum-dependent ImMDI interac-tion together with an isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model IBUU11,we have investigated relative effects of the density dependence of nuclear symmetry energy Esym(?)and the neutron-proton effective mass splitting mn*-mn*on the neutron/proton ratio of free nucleons and those in light clusters.It is found that the mn*-mp*has a relatively stronger effect than the Esym(?)and the assumption of mn*? mp*leads to a higher double neutron/proton ratio.More-over,this finding is independent of the in-medium nucleon-nucleon cross sections used.However,results of our calculations using the Esym(?)and mn*-mp*both within their current uncertainty ranges are all too low compared to the recen-t NSCL/MSU double neutron/proton ratio data from central 124Sn+124Sn and 112Sn+112Sn collisions at 50 and 120 MeV/u.We also introduce short-range cor-relation to explain the phenomenon that the results of our calculations is lower than the experimental data.Moreover,we calculate the kinetic energy difference between 3He and tritons and yield of 3He and tritons in 197Au+197Au collisions at the beam energy of 250 AMeV.It is seen that when the neutron's effective mass is larger than the proton's effective mass,the average kinetic energy of 3He is larger than the triton's average kinetic energy.When the neutron's effective mass is less than the proton's effective mass,the result is opposite.The experimental data shows that the average kinetic energy of 3He is larger than the triton.Thus,the effective mass splitting of mn*>mp*is more reasonable comparing with the experimental data.However,the difference of 3He's average kinetic energy and triton's average kinetic energy is smaller comparing with the experimental data.In addition,the number of 3He per event is larger than triton.Finally,with a newly improved isospin-and momentum-dependent interac-tion and an isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model,we have investigated the effects of the slope parameter L of the nuclear sym-metry energy and the isospin splitting of the nucleon effective mass mn-p*=(mn*-mp*)/m on the centroid energy of the isovector giant dipole resonance and the electric dipole polarizability in 208Pb.With the isoscalar nucleon effective mass ms*= 0.7m constrained by the empirical optical potential,we obtain a con-straint of L = 64.29±11.84(MeV)and mn-p*=(-0.019±0.090)?,with ? being the isospin asymmetry of nuclear medium.With the isoscalar nucleon effective mass ms*= 0.84m extracted from the excitation energy of the isoscalar giant quadruple resonance in 208Pb,we obtain a constraint of L = 53.85±10.29(MeV)and mn-p*=(0.216±0.114)?.