Disentangling sources of momentum fluctuations in Xe+Xe and Pb+Pb collisions with the ATLAS detector

TL;DR

This paper introduces a new method to separate geometrical and intrinsic fluctuations in high-energy nuclear collisions, providing insights into the initial conditions and properties of the quark-gluon plasma using ATLAS detector data.

Contribution

The study presents a novel technique to distinguish geometrical from intrinsic fluctuations in collision data, enhancing understanding of quark-gluon plasma properties.

Findings

Distinct fluctuation behaviors in ultra-central collisions.

Demonstration of a new disentangling technique.

Constraints on initial conditions and plasma properties.

Abstract

High-energy nuclear collisions create a quark-gluon plasma, whose initial condition and subsequent expansion vary from event to event, impacting the distribution of the event-wise average transverse momentum ($P([p_{\mathrm{T}}])$). Distinguishing between contributions from fluctuations in the size of the nuclear overlap area (geometrical component) and other sources at fixed size (intrinsic component) presents a challenge. Here, these two components are distinguished by measuring the mean, variance, and skewness of $P([p_{\mathrm{T}}])$ in $^{208}$Pb+$^{208}$Pb and $^{129}$Xe+$^{129}$Xe collisions at $\sqrt{s_{{\mathrm{NN}}}} = 5.02$ and 5.44 TeV, respectively, using the ATLAS detector at the LHC. All observables show distinct changes in behavior in ultra-central collisions, where the geometrical variations are suppressed as the overlap area reaches its maximum. These results demonstrate a new technique to disentangle geometrical and intrinsic fluctuations, enabling constraints on initial condition and properties of the quark-gluon plasma, such as the speed of sound.