Characterizing the initial conditions of heavy-ion collisions at the LHC with mean transverse momentum and anisotropic flow correlations

Correlations between mean transverse momentum $[p_{\rm T}]$ and anisotropic flow coefficients $v_{\rm 2}$ or $v_{\rm 3}$ are measured as a function of centrality in Pb-Pb and Xe-Xe collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV and 5.44 TeV, respectively, with ALICE. In addition, the recently proposed higher-order correlation between $[p_{\rm T}]$, $v_{\rm 2}$, and $v_{\rm 3}$ is measured for the first time, which shows an anticorrelation for the presented centrality ranges. These measurements are compared with hydrodynamic calculations using IP-Glasma and $\rm T_{R}ENTo$ initial-state shapes, the former based on the Color Glass Condensate effective theory with gluon saturation, and the latter a parameterized model with nucleons as the relevant degrees of freedom. The data are better described by the IP-Glasma rather than the $\rm T_{R}ENTo$ based calculations. In particular, Trajectum and JETSCAPE predictions, both based on the $\rm T_{R}ENTo$ initial state model but with different parameter settings, fail to describe the measurements. As the correlations between $[p_{\rm T}]$ and $v_{\rm n}$ are mainly driven by the correlations of the size and the shape of the system in the initial state, these new studies pave a novel way to characterize the initial state in relativistic heavy-ion collisions.


Submitted to: PRL
e-Print: arXiv:2111.06106 | PDF | inSPIRE

Figure 1

Centrality dependence of $\rho \left(v_{\rm 2}^{2}, [ p_{\rm T} ] \right)$ (left) and $\rho \left(v_{\rm 3}^{2}, [ p_{\rm T} ] \right)$ (right) in Pb--Pb at $\sqrt{s_{\rm NN}} = 5.02$~TeV (a, c) and Xe--Xe collisions $\sqrt{s_{\rm NN}} = 5.44$~TeV (b, d). The statistical (systematic) uncertainties are shown as vertical bars (filled boxes). The initial-state estimations (ISE) are represented by lines, while IP-Glasma+MUSIC+UrQMD~, v-USPhydro~, Trajectum~, and JETSCAPE~ hydrodynamic model calculations are shown with hatched bands.

Figure 2

Centrality dependence of $\rho \left(v_{\rm 2}^{2}, v_{\rm 3}^{2}, [ p_{\rm T} ] \right)$ in Pb--Pb collisions, shown by the solid circles. The statistical (systematic) uncertainties are shown as vertical bars (boxes). The initial-state estimations (ISE) as well as IP-Glasma+MUSIC+UrQMD~ and v-USPhydro~ hydrodynamic model calculations are compared with data.