Initial-state fluctuations from midperipheral to ultracentral collisions in an event-by-event transport approach

Abstract

We have developed a relativistic kinetic transport approach that incorporates initial-state fluctuations allowing to study the buildup of elliptic flow ${v}_{2}$ and high-order harmonics ${v}_{3}$, ${v}_{4}$, and ${v}_{5}$ for a fluid at fixed $\ensuremath{\eta}/s(T)$. We study the effect of the $\ensuremath{\eta}/s$ ratio and its $T$ dependence on the buildup of the ${v}_{n}({p}_{T})$ for two different beam energies: RHIC for Au+Au at $\sqrt{s}=200\phantom{\rule{0.28em}{0ex}}\text{GeV}$ and LHC for $\text{Pb}+\text{Pb}$ at $\sqrt{s}=2.76\phantom{\rule{0.16em}{0ex}}\text{TeV}$. We find that for the two different beam energies considered the suppression of the ${v}_{n}({p}_{T})$ due to the viscosity of the medium have different contributions coming from the crossover or QGP phase. Our study reveals that only in ultracentral collisions $(0--0.2%)$ the ${v}_{n}({p}_{T})$ have a stronger sensitivity to the $T$ dependence of $\ensuremath{\eta}/s$ in the QGP phase and this sensitivity increases with the order of the harmonic $n$. Moreover, the study of the correlations between the initial spatial anisotropies ${\ensuremath{\epsilon}}_{n}$ and the final flow coefficients ${v}_{n}$ shows that at LHC energies there is more correlation than at RHIC energies. The degree of correlation increases from peripheral to central collisions, but only in ultracentral collisions at LHC, we find that the linear correlation coefficient $C(n,n)\ensuremath{\approx}1$ for $n=2,3,4$, and 5. This suggests that the final correlations in the $({v}_{n},{v}_{m})$ space reflect the initial correlations in the $({\ensuremath{\epsilon}}_{n},{\ensuremath{\epsilon}}_{m})$ space.