Investigating initial state of heavy-ion collisions using [ <i>p</i> _T ] fluctuations and <i>v</i> _<i>n</i> − [ <i>p</i> _T ] correlations in ATLAS

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( v 2 , [ p T ]) between the two systems in the ultra-central region suggests that 129 Xe has large quadrupole deformation with a significant triaxiality. The measurement of v n −[ p T ] correlation provides the first measurement of triaxiality in 129 Xe using heavy ion collisions and provides new constraints to current models which fail to describe many of the observed trends in data.