Thermodynamics of hot strong-interaction matter from ultrarelativistic nuclear collisions

TL;DR

This paper uses advanced hydrodynamic simulations and experimental data from LHC lead-lead collisions to determine the temperature, entropy density, and speed of sound in quark-gluon plasma, confirming its deconfined phase.

Contribution

It provides the first experimental determination of thermodynamic quantities like temperature and entropy density in heavy-ion collisions using state-of-the-art simulations.

Findings

Measured temperature and entropy density in quark-gluon plasma.

Results agree with lattice QCD calculations.

Confirmed production of deconfined phase of matter.

Abstract

Collisions between heavy atomic nuclei at ultra-relativistic energies are carried out at particle colliders to produce the quark-gluon plasma, a state of matter where quarks and gluons are not confined into hadrons, and colour degrees of freedom are liberated. This state is thought to be produced as a transient phenomenon before it fragments into thousands of particles that reach the particle detectors. Despite two decades of investigations, one of the big open questions is to obtain an experimental determination of the temperature reached in a heavy-ion collision, and a simultaneous determination of another thermodynamic quantity, such as the entropy density, that would give access to the number of degrees of freedom. Here we obtain the first such determination, utilizing state-of-the-art hydrodynamic simulations. We define an effective temperature, averaged over the space-time evolution of the medium. Then, using experimental data, we determine this temperature, the corresponding entropy density and speed of sound in the matter created in lead-lead collisions at the Large Hadron Collider. Our results agree with first-principles calculations from lattice quantum chromodynamics and confirm that a deconfined phase of matter is indeed produced.