Influence Of The Electromagnetic Field On The Pion Observable In The Heavy-ion Collision

Heavy ion collisions can directly produce nuclear matter with high density and high temperature,and thus become an important research method to study the equation of state of nuclear matter and its symmetric energy term,a hot issue in the field of nuclear physics.In heavy ion collisions,in addition to observations such as neutron-proton ratio and nuclear collective flow,?-ions and?^-/?⁺ratio produced in the reaction have also become sensitive observations to detect nuclear matter equation of state,especially symmetry energy.In the past,in the case of relatively low beam energy,the transport model simulating heavy ion collisions generally used electrostatic Coulomb field to calculate the electromagnetic field effect produced by charged particles.However,with the advancement of accelerator technology,the beam energy is also increasing,and the nucleon in the collision nucleus has been accelerated to a high enough speed,such as the ¹⁹⁷Au+¹⁹⁷Au collision experiment on the FOPI accelerator.When the beam energy is 400 Me V/nucleon,the nucleon velocity is nearly 0.7 times the speed of light.Therefore,it is necessary to consider the relativistic effect in calculating the electromagnetic field effect generated by charged particles.In view of this,some heavy ion transport models consider electromagnetic calculations based on the Liénard-Wiechert potential.Compared with the static Coulomb field,this calculation scheme can not only correctly consider the relativistic effect on the electric field component,but also reasonably give the calculation of the magnetic field.However,when calculating the electromagnetic field based on this scheme,it is often assumed that the influence of the radiation field on the charged particles can be ignored,and because the calculation of the radiation field is more complex and time-consuming than the calculation of the velocity field,the vast majority of the transport models only use the simplified Liner-Weischer potential,thus only considering the velocity field.However,there is no quantitative research on the calculation of electromagnetic field to reveal whether the radiation field can be ignored.At the same time,there is no research to reveal the beam energy range applicable to the simplified Liner-Weischer potential,that is,the beam energy dependence of electromagnetic field calculation.In view of this situation,based on the Isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU)model,this thesis systematically studied the influence of different calculation schemes of electromagnetic field in medium energy heavy ion collisions on the hot issue of detecting the symmetric energy of nuclear matter.On the calculation of electromagnetic field,this thesis starts from Maxwell equation.Firstly,the electromagnetic field calculation scheme based on Maxwell equation is given,which is self-consistent calculation scheme;Secondly,under the point-particle approximation,this thesis gives a complete Lena-Weissell potential based on the self-consistent electromagnetic field calculation scheme;Then,through the non-relativistic approximation of the Lena-Weissell potential,the static coulomb field formula for calculating the electric field is given.In order to reveal the influence of different electromagnetic field calculation schemes on the observation of?meson and?^-/?⁺ratio,this thesis systematically simulates the two identical reaction systems ⁹⁶Ru+⁹⁶Ru and⁹⁶Zr+⁹⁶Zr under different beam energy,in order to reveal the influence of electromagnetic field on the observation of?meson and?^-/?⁺ratio and the dependence of beam energy on the influence of electromagnetic field.Firstly,this thesis studies the multiplicity and?^-/?⁺ratio of?^-and?⁺in the central collision of ¹⁹⁷Au+¹⁹⁷Au when the beam energy is 400Me V/nucleon under the two calculation schemes of the static coulomb field and the commonly used simplified(ignoring the radiation/acceleration field)Liénard-Wiechert formula.Secondly,the dependence of the relativistic delay effect of the electric field on the collision parameters and the dependence of the beam energy were investigated.The double(?^-/?⁺)ratio of the two reaction systems ⁹⁶Ru+⁹⁶Ru and⁹⁶Zr+⁹⁶Zr under different beam energy were also investigated.Finally,the difference of?meson yield and?^-/?⁺ratio of ⁹⁶Ru+⁹⁶Ru central collision under three calculation schemes of electromagnetic field with beam energy of 400 Me V/nucleon is studied.The three calculation schemes are self-consistent calculation,the most commonly used SLW formula and the static coulomb field formula.The results show that the commonly used simplified Liénard-Wiechert formula is not accurate enough to calculate the electromagnetic field,because the radiation field ignored in this formula also greatly affects the yield of charged?mesons and their?^-/?⁺ratio.Therefore,it is suggested that the calculation of electromagnetic field should be carefully considered when detecting the symmetry energy of nuclear matter based on heavy ion collisions and using?meson observations.In addition,this thesis further analyzed the influence of the double?^-/?⁺ratio of the two reaction systems ⁹⁶Ru+⁹⁶Ru and ⁹⁶Zr+⁹⁶Zr under the electromagnetic field at 400 Me V/nucleon.The results show that the double?^-/?⁺ratio of the two reactions is often less affected by the electromagnetic field,so it can still be used as an effective observation of nuclear symmetry energy in heavy ion collisions.