Extracting the speed of sound in the strongly interacting matter created in relativistic nuclear collisions with the CMS experiment

TL;DR

This paper measures the speed of sound in hot, dense matter created in ultrarelativistic nuclear collisions using CMS data, providing insights into the matter's equation of state and QCD phase transition.

Contribution

It presents a novel extraction of the speed of sound from CMS lead-lead collision data, linking experimental observables to theoretical QCD predictions.

Findings

Measured speed of sound consistent with hydrodynamic models

Results align with lattice QCD predictions at high temperatures

Implications for QCD phase transition and critical point search

Abstract

A hot and dense matter exhibiting collective flow behavior with almost no viscous dissipation has been discovered in ultrarelativistic nuclear collisions. To constrain the fundamental degrees of freedom and equation of state of this matter, these proceedings present an extraction of its speed of sound using head-on lead-lead collision data collected by the CMS experiment at a center-of-mass energy per nucleon pair of 5.02 TeV. The measurement is based on an analysis of the observed charged multiplicity dependence of the average particle transverse momentum in ultra-central events (impact parameter of nearly zero), a variable which probes the system temperature as a function of entropy density at a fixed volume. Results are compared with hydrodynamic simulations and lattice quantum chromodynamics (QCD) predictions of the equation of state at high temperatures and small chemical potential. Implications to search for QCD phase transition and the critical point are discussed.