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Effects of hydrodynamic and initial longitudinal fluctuations on rapidity decorrelation of collective flow

We investigate the interplay between hydrodynamic fluctuations and initial longitudinal fluctuations for their effects on the rapidity decorrelation of collective flow in high-energy nuclear collisions. We use a (3+1)-dimensional integrated dynamical model in which we combine initial conditions with longitudinal fluctuations, fluctuating hydrodynamics and hadronic cascades. We analyse the factorisation ratio in the longitudinal direction to study the effect of these fluctuations on the rapidity decorrelation. We find an essential difference between the effects of the hydrodynamic fluctuations and the initial longitudinal fluctuations in the centrality dependence of the factorisation ratios. A combination of the hydrodynamic fluctuations and the initial longitudinal fluctuations leads to reproduction of the centrality dependence of the second-order factorisation ratio, $r_2(η_\mathrm{p}^\mathrm{a},η_\mathrm{p}^\mathrm{b})$, measured by the CMS Collaboration. Our model also qualitatively describes the centrality dependence of the third-order factorisation ratio, $r_3(η_\mathrm{p}^\mathrm{a},η_\mathrm{p}^\mathrm{b})$. These results demonstrate the importance of the hydrodynamic fluctuations, as well as the initial longitudinal fluctuations, in understanding the longitudinal dynamics of high-energy nuclear collision reactions.