Transverse and longitudinal event-by-event flow fluctuations of $v_1$−$v_4$ in 5.02 TeV Pb+Pb collisions with the ATLAS detector

Multi-particle flow correlations in Pb+Pb collisions provide unique insight into the nature of event-by-event fluctuations of the initial eccentricity as well as final state dynamics in the transverse and longitudinal directions. This talk presents a detailed study of transverse flow fluctuations using 4 and 6-particle cumulants $v_n\{4\}$ and $v_n\{6\}$ for $n=1,2,3$ and 4. This includes several new results: the first measurement of a negative dipolar flow $v_1\{4\}$; a high-precision measurement of $v_4\{4\}$, changing sign around 20-25$\%$ centrality; observation of an intriguing sign-change pattern of $v_2\{4\}$ and $v_2\{6\}$ in ultra-central collisions; a detailed study of the cumulant ratio $v_n\{6\}/v_n\{4\}$ which shows significant deviation of $v_2$ and $v_3$ from both Bessel-Gaussian and elliptic-power distributions. The three-subevent cumulant method is used to show that these results are unlikely to be due to non-flow effects. The talk also presents a detailed study of the longitudinal dynamics of harmonic flow using various correlators involving two, four or six particles. The flow decorrelations for vn ($n=2,3$, and 4), as well as their center-of-mass energy dependence are studied over broad range of pseudorapidity ($|\eta|<2.5$) and transverse momentum ($0.5<p_T<5$ GeV). The decorrelation signals are decomposed into contributions from the forward-backward twist and asymmetry in the flow angle and magnitude, respectively. Furthermore, the decorrelation between $v_n$ and $v_m$ in different $\eta$ is measured to disentangle the longitudinal dependence of the initial-state linear effects and final-state non-linear mode-mixing effects. These results provide a wealth of differential information on event-by-event fluctuations of harmonic flow, and they can be used to improve event-by-event 3+1D hydrodynamic models.