Hydrodynamic fluctuations and ultra-central flow puzzle in heavy-ion collisions

One of the long-standing problems in the field of high-energy heavy-ion collisions is that the dynamical models based on viscous hydrodynamics fail to describe the experimental elliptic flow v2 and the triangular flow v3 simultaneously in ultra-central collisions. The problem, known as the ultra-central flow puzzle, is specifically that hydrodynamics-based models predict the flow ratio of the two-particle cumulant method v2{2}/v3{2} > 1 while v2{2}/v3{2} -1 in the experimental data. In this Letter, we focus on the effects of hydrodynamic fluctuations during the space-time evolution of the QGP fluid on the flow observables in the ultra-central collisions. Using the (3+1)-dimensional integrated dynamical model which includes relativistic fluctuating hydrodynamics, we analyze the anisotropic flow coefficients vn{2} in 0-0.2% central Pb+Pb collisions at & RADIC;sNN = 2.76 TeV. We find that the hydrodynamic fluctuations decrease the model overestimate of v2{2}/v3{2} from the experimental data by about 19% within the present setup of & eta;/s =1/2 & pi;. This means that the hydrodynamic fluctuations qualitatively have an effect to improve the situation for the puzzle, but the effect of the hydrodynamic fluctuations alone is quantitatively insufficient to resolve the puzzle. The decrease of the ratio largely depends on the shear viscosity & eta;/s, which calls for future comprehensive analyses with, for example, a realistic temperature -dependent viscosity.& COPY; 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/). Funded by SCOAP3.

Automated summary

One of the long-standing problems in high-energy heavy-ion collisions is the failure of viscous hydrodynamics-based dynamical models to simultaneously describe the experimental elliptic flow v2 and the triangular flow v3 in ultra-central collisions. This is known as the ultra-central flow puzzle, specifically the discrepancy between the predicted flow ratio of the two-particle cumulant method v2{2}/v3{2} > 1 and v2{2}/v3{2} -1 in experimental data. In this study, the effects of hydrodynamic fluctuations during the space-time evolution of the QGP fluid on flow observables in ultra-central collisions are analyzed, revealing a qualitative improvement in the situation for the puzzle but a quantitative insufficiency in resolving it.