Relative Yield Of π~± And Time-Space Characteristic Of The Source In 1AGeV Au+Au Collsions

This thesis simulates 1AGeV Au+Au central collision with the isospin quantum molecular dynamics(IQMD) approach and presents a detailed discussion of the production machanism of pions as well as coulomb effect on the relative yield ofπ~±mesons.The IQMD model is introduced in detail at first, then shows the production machanism of pions and the generation machanism of concave pion spectra at length. Coulomb force is a kind of long range force, this makes it also influences the particles after freeze-out. This brings extra trouble in understanding the time-space characteristic of the source.This thesis uses the IQMD model and analyzes the relationship between frozen energy and frozen time, frozen position and frozen time as well the frozen position and frozen energy . Then obtains the size of the collision source with the relative yield ofπ~±mesons. We do a detailed analyzition for the low-energy pions and find that the value of relative yield ofπ~±mesons is arise when energy increased. when the kinetic energy is up to 27MeV, the value reach a utmost value, then decends and down to a constant. we see a peak in the figure ofπ~-/π~+ varies with energy. By comparing the average value of the angle's cosine between momentum and radius direction of low-energy pions at freeze out time and final state respectively, we find that the transmition ofπ~±mesons at freeze-out time is nearly isotropic, but towards out along the radius at final state.Using the average velocity of protons and the average velocity of low-energy pions as well as the velocity of pions at peak, we get such conclusion that the influence of the coulomb effect is different on high-energy pions and low-energy pions. Therefore, we explain the high peak phenomenon which is found in the figure ofπ~-/π~+ varies with energy at final state.