Orateur
Description
The speed of sound squared, $c_s^2$, one of the properties of the quark-gluon plasma (QGP) connected to the QCD equation of state, can be extracted from ultra-central heavy-ion collisions, where the medium mostly maintains a fixed size and fluctuations in the initial state and thermal fluctuations dominate. We present the first ALICE measurements of the event-by-event mean transverse momentum, $\langle[p_\mathrm{T}]\rangle$, in particular its average and higher-order fluctuations as a function of multiplicity using particle spectra and multi-particle $\langle[p_\mathrm{T}]\rangle$ cumulant techniques, in ultra-central Pb--Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV. The pronounced rise in $\langle[p_\mathrm{T}]\rangle$ and the sudden transition in higher-order fluctuations at high multiplicities are used to extract the speed of sound in QGP, $c_s^2$, and to probe the thermalisation of the QGP, respectively. Our approach yields valuable insights into the thermalized nature of the deconfined state resulting from heavy-ion collisions, contributing to a deeper understanding of the QCD equation of state.
