Exploring the origin of $[p_\mathrm{T}]$ fluctuations in ultra-central heavy ion collisions: Higher order $[p_\mathrm{T}]$ correlations in ATLAS

The thermal fluctuations in the QGP medium formed in heavy ion collisions presents itself as final state $[p_\mathrm{T}]$ fluctuations. Measurement of $[p_\mathrm{T}]$ in ultra-central collisions can help differentiate the interplay between the effect of radial collectivity, random thermal motion and deformation in nuclear geometry. Recent studies have shown that event-by-event $[p_\mathrm{T}]$ correlations in ultra-central heavy ion collisions can provide valuable information about the thermalization of the QGP droplet. This talk presents new precise measurements of $[p_\mathrm{T}]$ cumulants up to 3rd order in $^{129}$Xe+$^{129}$Xe and $^{208}$Pb+$^{208}$Pb collisions. On an average, all orders of $[p_\mathrm{T}]$ cumulants show deviations from expected power-law behavior. The $[p_\mathrm{T}]$ shows a non-trivial rise in the ultra-central collisions, whereas the variance shows a clear and sharp drop in ultra-central collisions. The skewness, expected to be more sensitive towards higher p_\mathrm{T} particles, also shows non-trivial increase in it’s magnitude for ultra-central collisions. All observables show a clear dependence on the $p_\mathrm{T}$ ranges in consideration and the centrality estimator used in the analysis, indicating a strong influence of volume fluctuations. These results have strong implications on understanding the contribution of nuclear deformation, initial condition, medium thermalization and medium properties towards final state $[p_\mathrm{T}]$ fluctuations.