Flow and transverse momentum fluctuations in Pb+Pb and Xe+Xe collisions with ATLAS: assessing the initial condition of the QGP

Constraining the initial condition of the QGP using experimental observables is one of the most important challenges in our field. Recent studies show that the Pearson Correlation Coefficient (PCC) between vn and event-wise mean transverse momentum [pT], ρn(vn, [pT]), and [pT] fluctuations can probe several ingredients of the initial state. This talk presents precision measurements of vn − [pT] correlation for n=2,3 and 4 and high order [pT] fluctuations in 129Xe+129Xe and 208Pb+208Pb collisions, and they are found to be small in the mid-central and central collisions in these systems. The ρn and variance and skewness of [pT] fluctuations show non-monotonic dependence on centrality, pT and η. It was also found that the result depends on the centrality estimator used in the analysis, indicating a strong influence of volume fluctuations. In central collisions, where models generally show good agreement, the v2–[pT] correlations are sensitive to the triaxiality of the quadrupole deformation. A comparison of the model with the Pb+Pb and Xe+Xe data confirms that the 129Xe nucleus is a highly deformed triaxial ellipsoid that has neither a prolate nor oblate shape. This provides strong evidence for a triaxial deformation of the 129Xe nucleus from high-energy heavy-ion collisions.