Longitudinal fluctuations and decorrelations of anisotropic flows at energies available at the CERN Large Hadron Collider and at the BNL Relativistic Heavy Ion Collider

We perform a systematic study on the decorrelation of anisotropic flows along the pseudorapidity in relativistic heavy-ion collisions at the CERN Large Hadron Collider (LHC) and BNL Relativistic Heavy Ion Collider (RHIC) energies. The dynamical evolution of the quark-gluon plasma fireball is simulated via the CLVisc (ideal) (3 + 1)-dimensional hydrodynamics model, with the fully fluctuating initial condition from the A-Multi-Phase-Transport (AMPT) model. A detailed analysis is performed of the longitudinal decorrelations of elliptic, triangular, and quadrangular flows in terms of flow vectors, flow magnitudes, and flow orientations (event planes). It is found that pure flow magnitudes have a smaller longitudinal decorrelation than pure flow orientations, and the decorrelation of flow vectors is a combined effect of both flow magnitudes and orientations. The longitudinal decorrelation of elliptic flow has a strong and nonmonotonic centrality dependence due to the initial elliptic collision geometry: the smallest decorrelation in midcentral collisions. In contrast, the decorrelations of triangular and quadrangular flows have a weak centrality dependence, with slightly larger decorrelations in more peripheral collisions. Our numerical results for Pb Pb collisions at the LHC are in good agreement with the ATLAS data, while our RHIC results predict much larger longitudinal decorrelations compared to the LHC. We further analyze the longitudinal structures of the AMPT initial conditions and find that the final-state longitudinal decorrelation effects are strongly correlated with the lengths of the initial string structures in the AMPT model. The decorrelation effects are typically larger at lower collision energies and in more peripheral collisions due to the shorter lengths of the string structures in the initial states.