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

One important challenge in our field is to understand the initial condition of the QGP and constrain it using sensitive experimental observables. Recent studies show that the Pearson Correlation Coefficient (PCC) between vn and event-wise mean transverse momentum [pT], rho(vn, [pT]), and its centrality dependence can probe several ingredients of the initial state, such as number and size of sources, nuclear deformation, volume fluctuation, and initial momentum anisotropy. In particular, a recent calculation shows that the 129Xe nucleus is triaxially deformed, which is expected to enhance rho in 129Xe+129Xe relative to 208Pb+208Pb collisions. This talk presents comprehensive and precision measurements of vn-pT correlation in 129Xe+129Xe and 208Pb+208Pb collisions for harmonics n = 2, 3, and 4. The results are obtained via the standard and subevents cumulant methods to assess the role of non-flow and flow decorrelations in these observables, and they are found to be small in the mid-central and central collisions in these systems. All PCC coefficients, rho_2, rho_3 and rho_4 show rich and non- monotonic dependence on centrality, pT and eta, reflecting the fact that different ingredients of the initial state impact different regions of the phase space. For example, we found the result depends on the centrality estimator used in the analysis, indicating a strong influence of volume fluctuations. On the other hand, the ratio of rho_n between the two systems is less sensitive to the centrality estimator, and in the ultra-central region, the value of the ratio suggests that 129Xe has large quadrupole deformation but with a significant triaxiality. All current models fail to describe many of the observed trends in the data, pointing to the unprecedented constraining power enabled by this precision measurement.