Global Polarization Of ?-mesons And ? Hyperons In Heavy Ion Collisions With AMPT Model

In non-central relativistic heavy ion collisions,the created matter possesses a large initial orbital angular momentum,Particles produced in the collisions could be polarized globally in the direction of the orbital angular momentum due to spin-orbit coupling.Recently,the STR experiment has presented the polarization signals for hyperons and possible spin alignment for ? mesons,which indicates the QGP created in the experiment may be the most vertical fluid in nature.These results opened a new direction for better understanding the medium properties in such collisions.Due to the limitation of STR experimental acceptance and small measured value of hyperons polarization parameter,it is important to study the acceptance effect on the measurements.In this thesis,we will use a multi-phase transport model that includes the degree of hyperons polarization and ? mesons spin alignment signal to study the finite acceptance effect on the global polarization observables ?00 and pH.We calculate angular momentum of system event-by-event.? meson spin alignment is described by its spin density matrix p,which is a 3×3 matrix. deviation of the diagonal element from 1/3 is regarded as the signal of spin alignment.We found the hadronic scatterings after?-meson decays are irrelevant for ?-meson spin alignment.But the finite pseudorapidity coverage has large effect on ?-meson spin alignment parameter ?00. narrower pesudorapidity acceptance gives a significantly larger ?00 value than the input ?00.We also compared with experimental results at 7.7Gev,19.6GeV,62.4GeV and 200GeV by using the same acceptance cuts.We found these model results are similar to the preliminary experimental data,suggesting that the acceptance cuts may be a dominant contribution to the observed higher-than-1/3 value. Toy Model was used to study the finite pseudorapidity coverage effect on polarization parameter pH.We found the narrower pseudorapidity acceptance gives a larger pH value than the input pH.But assuming no polarization(pH=0),the finite pseudorapidity coverage has no effect on extraction of polarization parameter.By setting a constant input pH as a function of collisions energy,no energy dependence is observed in the model results with same acceptance cuts of experiment.Our study suggests that the measured polarization observables needs to be corrected for the effects of finite acceptance before quantitative conclusion on global polarization can be reliably made.