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 $v_{n}$ and event-wise mean transverse momentum $[p_{\mathrm{T}}]$, $\rho_{n}(v_{n},[p_{\mathrm{T}}])$ can probe number and size of sources, nuclear deformation, volume fluctuation, and initial momentum anisotropy in initial state of heavy-ion collisions. This talk presents new, comprehensive and precision measurements of $v_{n}-[p_{\mathrm{T}}]$ correlation using $^{\mathrm{129}}\mathrm{Xe}+^{\mathrm{129}}\mathrm{Xe}$ and $^{\mathrm{208}}\mathrm{Pb}+^{\mathrm{208}}\mathrm{Pb}$ collisions for harmonics $n=2$, 3, and 4 at LHC using ATLAS detector. $\rho_{n}$ shows rich and non-monotonic dependence on centrality, $p_{T}$ and $\eta$, reflecting the fact that different ingredients of the initial state impact different regions of the phase space. The ratio of $\rho_{2}$ between the two systems in the ultra-central region suggests that ${^\mathrm{129}}\mathrm{Xe}$ 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.