Investigating initial state of heavy-ion collisions using [p_T] fluctuations and v_n − [p_T] correlations in ATLAS

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Published online: 5 November 2025

Abstract

This proceeding presents recent studies of event-wise mean transverse momentum, [p_T] that can help differentiate the interplay between the effect of radial collectivity, random thermal motion and deformation in nuclear geometry. In addition, the Pearson Correlation Coefficient (PCC) between flow, v_n and [p_T], ρ(v_n, [p_T]), will be shown. The results bear on aspects of the initial state, such as nuclear deformation and initial momentum anisotropy. New precise ATLAS measurements of [p_T] cumulants up to 3rd order and v_n − [p_T] correlations in Xe+Xe, Pb+Pb collisions are presented. These measurements provide the first experimental handle to isolate initial state and medium evolution contributing to final state momentum fluctuations. The PCC coefficients show a non-monotonic dependence on centrality, [p_T] and η, reflecting the fact that different aspects of the initial conditions affect different regions of the phase space. The ratio of ρ(v₂, [p_T]) between the two systems in the ultra-central region suggests that ¹²⁹Xe has large quadrupole deformation with a significant triaxiality. The measurement of v_n−[p_T] correlation provides the first measurement of triaxiality in ¹²⁹Xe using heavy ion collisions and provides new constraints to current models which fail to describe many of the observed trends in data.