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

TL;DR

This paper presents precise measurements of the correlation between flow coefficients and mean transverse momentum in Xe+Xe and Pb+Pb collisions, providing insights into the initial conditions of the quark-gluon plasma and challenging existing models.

Contribution

It offers new experimental data on ${m V}_n$-${p_{ ext{T}}}$ correlations in heavy-ion collisions, highlighting complex dependencies and suggesting deformation effects in Xe nuclei.

Findings

Non-monotonic dependence of correlation on centrality, $p_T$, and $\\eta$

Evidence of large quadrupole deformation and triaxiality in Xe nuclei

Current models do not fully reproduce observed trends

Abstract

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 between ${\bm V}_n$ and event-wise mean transverse momentum $[p_{\mathrm{T}}]$, $\rho_{n}({\bm V}_n,[p_{\mathrm{T}}])$ can probe number and size of sources, nuclear deformation, volume fluctuation, and initial momentum anisotropy in initial state of heavy-ion collisions. This proceeding presents new, precision measurements of ${\bm V}_n-[p_{\mathrm{T}}]$ correlation in $^{\mathrm{129}}\mathrm{Xe}+^{\mathrm{129}}\mathrm{Xe}$ and $^{\mathrm{208}}\mathrm{Pb}+^{\mathrm{208}}\mathrm{Pb}$ collisions for harmonics $n=2$, 3, and 4 using the ATLAS detector at the LHC. The values of $\rho_{n}$ shows rich and non-monotonic dependence on centrality, $p_{T}$ and $\eta$, reflecting 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 and with a significant triaxiality. All current models fail to describe many of the observed trends in the data.