Speaker
Description
The speed of sound squared, $c_s^2$, a property 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 maintains a fixed size and the initial-state and thermal fluctuations dominate. We present the first ALICE measurements of the event-by-event mean transverse momentum, $\langle[p_\mathrm{T}]\rangle$, 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 $c_s^2$ and to probe the thermalization of the QGP. Our approach yields valuable insights into the thermalized nature of the QGP, contributing to a deeper understanding of the QCD equation of state.
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