Measurement of Transverse Momentum Fluctuations in Xe+Xe and Pb+Pb Collisions with ATLAS

Measurements of the first three cumulants of the distribution of the event-by-event transverse momentum distribution, including the mean $\langle[\pT]\rangle$, variance, and skewness, are performed with the ATLAS detector at the Large Hadron Collider. These measurements are carried out for Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44$ TeV with an integrated luminosity of 3 $\mu\mathrm{b}^{-1}$ and Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with an integrated luminosity of 470 $\mu\mathrm{b}^{-1}$. The variance and skewness exhibit an approximate power-law behavior as functions of the charged particle multiplicity $N_{ch}^{rec}$, consistent with an independent superposition scenario. However, in the ultra-central collisions (UCC) region, these cumulants show a significant departure from their behavior observed in other centrality ranges. This departure is predicted to arise due to the suppression of impact parameter fluctuations and the dominance of intrinsic fluctuations associated with the initial energy deposition and final state system evolution. Notably, the rise of $\langle[\pT]\rangle$ in the UCC region is consistent with the presence of a thermalized medium predicted by model calculations. These measurements offer fresh insights into the nature of initial-state fluctuations and the properties of the quark-gluon plasma formed in heavy-ion collisions.