Longitudinal Fluctuations And Decorrelations Of Anisotropic Flows In Heavy-ion Collisions At The LHC And RHIC

High-energy heavy-ion collisions,performed at the Relativistic Heavy Ion Col-lider(RHIC)and the Large Hadron Collider(LHC),can create a high tempera-ture and dense environment which provides an opportunity for studying the strong-interaction nuclear matter.Various experiential results and theoretical studies have demonstrated that the extremely hot and dense nuclear matter is a strongly-coupled quark-gluon plasma(QGP).One of the most important evidences for the existence of QGP is the strong collective flow of QGP fireball which originates from the pres-sure gradient of the fireball and the strong interaction among the QGP constituents.The successfulness of relativistic hydrodynamics in describing the collective flow of QGP implies that QGP behaves like a relativistic fluid with extremely low shear viscosity-to-entropy density ratio.Fluctuations play a key role in understanding the origins of anisotropic collective flows and the transport properties of the hot and dense QGP.Because of the event-by-event fluctuations of the initial-state energy density and geometry,one can observe the azimuthally anisotropic momentum distributions for final-state soft hadrons.Re-cently,much attention has been paid to the fluctuations along transverse plane,such as higher-order anisotropic flows and symmetric cumulants.However,fluctuations in the longitudinal direction are also important,and can lead to different anisotropic collective flows along longitudinal(rapidity)direction.In this thesis,we focus on the fluctuations along the longitudinal directions and their manifestations in the final state anisotropic flows.We use the CLVisc(ideal)(3+1)-dimensional hydrodynamics model with fully fluctuating initial condition from the A-Multi-Phase-Transport(AMPT)model to simulate the space-time evolution of QGP.We perform a detailed analysis about the longitudinal decorrelations of el-liptic,triangular,and quadrangular flows,in terms of flow vectors,flow magnitudes,and flow angle orientation.It is found that the flow angle has a larger longitudinal decorrelation effect than flow magnitudes,and the longitudinal decorrelation of flow vectors is a combinational contribution from flow magnitude and flow orientation.It also found that there is a strong and non-monotonic centrality dependence for the longitudinal decorrelation of elliptic flow due to the initial elliptic collision geometry.In contract,the decorrelations of triangular and quadrangular flows have a weak cen-trality dependence with the slightly larger decorrelations in more peripheral collisions.Our numerical results nicely reproduce the ATLAS data for Pb+Pb collisions at the LHC.At the same time,our study predicts much larger longitudinal decorrelations for RHIC heavy-ion collisions.We also investigate the longitudinal structures of the AMPT initial model and find strong correlation between the length of initial string structures in the AMPT model and final-state longitudinal decorrelation effects.The longitudinal decorrelation effects are typically larger at lower collision energies and in more peripheral collisions where the lengths of the string structures are shorter in the initial states.