Quark Polarization In Relativistic Heavy-ion Collisions

Polarization phenomenology in high energy physics is a powerful tool to study the properties of nucleon structure and strong interaction,especially a series of unexpected spin effects provide a lot for us to understand the nonperturbative properties of QCD.Take the hyperon polarization for example,it is the first unexpected spin effect which has been observed in experiments measuring hyperon polarization since 1970's.The data shows that the hyperon produced in unpolarized hadron-hadron or hadron-nucleus collisions is transversely polarized with respect to the production plane.The origin for such striking transverse hyperon polarization is still in debate.Taking into account nucleus-nucleus collisions instead of hadron-hadron or hadron-nucleus collisions,it has been suggested in the Ref.[1,2]that hyperon polarization could disappear due to the formation of QGP.However, there're more primary experiments knobs available in nucleus-nucleus collisions than in hadron-hadron or hadron-nucleus collisions,such as impact parameter or centrality cuts can be systematically varied and the direction of reaction plane can also be determined.Under such available experimental tools,it has been pointed out by Z.T.Liang and X.N.Wang in Ref.[3] that,in non-central heavy-ion collisions at high energy,there exists a global huge orbital angular momentum in the overlapping region,and such global orbital angular momentum can lead to a longitudinal fluid shear distribution representing local relative orbital angular momentum.The presence of such local orbital angular momentum of the partonic system at the early stage of non-central heavy-ion collisions can lead to a global polarization of quarks and anti-quarks[3]in the direction orthogonal to the reaction plane due to the spin-orbital coupling in Quantum Chromodynamics(QCD).Such polarization in the Ref.[3]is estimated by evaluating the polarization cross section in the impact parameter space.For simplicity,the calculation is processed by using a screened static potential model in the small angle approximation. The results show that it is complete possible that the initial orbital angular momentum can lead to quark polarization,which can be 30%-40%. Z.T.Liang and X.N.Wang further argue that such a global quark/antiquark polarization should have many observable consequences such as global hyperon polarization[3]and vector meson spin alignment[4],and give some advice on how to measure these interesting phenomenologies.Since such polarization is due to the initial global orbital angular momentum in non-central heavy ion collisions,in order to make it distinct from the hyperon transverse polarization with respect to the production plane in hadron-hadron or hadron-nucleus collisions,we'll just call it global polarization.Global polarization will open a new door to study the coupling mechanism between the orbital and spin angular momentum in QCD.In my dissertation,I have extended an earlier study in[3].Firstly,we have calculated the average local relative orbital angular momentum or longitudinal fluid shear with two extreme models:Landau fireball and Bjorken scaling model.In the Landau fireball model,we assumed a wounded nucleon model for local particle production with both the hard-sphere and Woods-Saxon nuclear distributions.The longitudinal fluid shears from Woods-Saxon distributions is similar to the one from hard-sphere distributions.In the Bjorken scaling scenario,we use HIJING Monte Carlo model to estimate the transverse shear of the rapidity distribution of the produced parton in heavy-ion collisions at the RHIC energy which will be used to estimate the longitudinal fluid shear in the local comoving frame of the plasma.We've found that the difference between Landau fireball and Bjorken scaling model is quite distinct,and will finally lead to quite different quark polarization.Also,I'll generalize the static potential model to the general quark-quark scattering process with HTL gluon propagator by thermal field theory.We'll deal with the case where both quarks are light quarks,as well one is light quark and the other is heavy quark.The final numerical result shows that the quark polarization is more complicated and sensitive as the function of the energy than that in Ref.[3].In the light quark case,from low energy to high energy,the polarization will increase quickly at low energy,reach a maximum peak and finally decrease towards zero at high energy.The magnitude of the polarization can reach 25%.In the heavy quark case,the shape is similar but the magnitude is less than in the light case.The polarization decreases with the mass of heavy quark but more softly than the results in Ref.[3]. In semi-peripheral Au+Au collisions at the RHIC energy,the final quark polarization is found to be small[P_q]＜0.04 in both Landau fireball and Bjorken scaling model in the weak coupling limit.The effect is expected to be near the peak in Landau fireball model at LHC.Besides,we'll consider how the polarization of thermal quark can be induced by jets.Still with the help of HTL gluon propagator,We find that the direction of such associated polarization depends on the location of thermal quarks from the jet and the polarization is almost a constant when the energy of jets is much higher than the temperature of hot bath.