Thermodynamics of hot strong-interaction matter from ultrarelativistic nuclear collisions

The quark-gluon plasma, in which quarks and gluons are deconfined, is a transient state created in collisions of heavy nuclei. By defining an effective temperature, this temperature and the system's entropy density and speed of sound are determined. Collisions between heavy atomic nuclei at ultrarelativistic 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 challenges(1) 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 such a determination, utilizing state-of-the-art hydrodynamic simulations(2). We define an effective temperature, averaged over the spacetime evolution of the medium. Then, using experimental data, we determine this temperature and 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(3) and confirm that a deconfined phase of matter is indeed produced.