Collisions between heavy atomic nuclei at ultra-relativistic energies are carried out at particle colliders to produce a state of matter where quarks and gluons, the color degrees of freedom, are not bound - the quark–gluon plasma. This state is thought to be produced as a transient phenomenon before it fragments into thousands of particles that reach the particle detectors. I show how the thermodynamic properties of this transient state can be reconstructed from the information collected in detectors: The matter created in lead–lead collisions at the Large Hadron Collider at CERN is found to reach a temperature as high as 2.6 trillion degrees, the highest ever recorded in the laboratory. The value of the entropy density at this temperature, estimated from experimental data, agrees quantitatively with first-principles calculations from quantum chromodynamics.
These results confirm that a deconfined phase of matter is indeed produced, in which sound waves propagate at half the speed of light.