Effects Of The Nucleon Effective Mass Splitting And Symmetry Energy On Intermediate Heavy Ion Collisions

The equation of state(EOS)of nuclear matter reflects the thermodynamic relationship between the energy or pressure of nucleons(protons and neutrons)and the density of nuclear matter,isospin asymmetry,as well as the temperature.It has been an important subject in the research of nuclear physics and astrophysics.Heavy ion collision is the only way to generate high-density nuclear matter on Earth,so heavy ion collision experiments provides the unique opportunity to study the EOS of the dense nuclear matter.The understanding of the EOS of the isospin asymmetry(the number of protons and the number of neutrons is different)nuclear matter hasn’t been comprehensive so far.And the biggest uncertainty comes from the density-dependent symmetry energy.In addition,the effective mass of the nucleon is a fundamental quantity characterizing the nucleon’s propagation in nuclear medium,and its value is generally smaller than that in the vacuum.It has been used to characterize the momentum or energy dependence of the single-particle potential.For the reason that the single-particle potential of proton and neutron might be different,which causes the neutron-proton effective mass splitting m_n~*-m_p~*.It is currently one of the hotspots in the field of nuclear physics.However,different signs of m_n~*-m_p~*is given by employing different theories.Thus,it is necessary to conduct more researches on the neutron-proton effective mass splitting and the nuclear symmetry energy.In this work,we updated the ultra-relativistic quantum molecular dynamics model (Ur QMD).The main improvement is that an isospin-dependent form of the momentum-dependent potential which makes the effective masses of protons and neutrons are different is introduced into the Ur QMD model.Four sets of parameters with different properties are given to study the effects of nucleon effective mass splitting and symmetry energy on obsevables in heavy ion collisions at intermediate energies.Taking the semicentral collisions of Au+Au with incident energy of 0.09Ge V/nucleon to 1.5 Ge V/nucleon as an example,the effects of different symmetry energy and effective mass splitting on particle yield,collective flow,transverse momentum distribution,etc are shown in this work.Thus,we can find some obsevables that are more sensitive to the extraction of symmetric energy and nucleon effective mass splitting.And the observations can provide references for experimental research.Finally we draw the conclusion that:(1)The approximate integral of the momentum-dependent potential makes the incompressible coefficient of EOS of the nuclear matter larger;(2)When the symmetry energy is relatively soft,the effects of nucleon effective mass splitting will be more evident.(3)The effect of symmetry energy can be better extracted in the range of incident energy from 0.6 to 1.0 Ge V/nucleon.However,when the energy is lower than 0.6 Ge V/nucleon,the effects of nucleon effective mass splitting and the nuclear symmetry energy on the elliptic flow difference between neutrons and protons are comparable with each other.