CERN researchers measure the speed of sound in quark-gluon plasma more precisely than ever before

Neutron stars in the universe, ultra-cold atomic gases in the laboratory and the quark-gluon plasma created during collisions of atomic nuclei at the Large Hadron Collider (LHC): they may seem completely unrelated but, surprisingly, they have something in common. They all constitute a fluid-like state of matter, composed of strongly interacting particles. Knowledge about the properties and behavior of each of these near-perfect liquids can be essential to understanding nature at very distant scales.

In a new paper, the CMS collaboration reports the most precise measurement yet of the speed at which sound travels in quark-gluon plasma, providing new insights into this extremely hot state of matter.

Sound is a longitudinal wave that passes through a medium, producing compressions and rarefactions of matter in the same direction as its movement. The speed of sound depends on the properties of the medium, such as its density and viscosity. It can therefore be used as a medium probe.

At the LHC, quark-gluon plasma is formed during collisions between heavy ions. In these collisions, for a very small fraction of a second, an enormous amount of energy is deposited in a volume the maximum size of which is that of the nucleus of an atom. Quarks and gluons emerging from the collision move freely in this region, creating a fluid-like state of matter whose collective dynamics and macroscopic properties are well described by theory.

The speed of sound in this environment can be obtained from the rate at which pressure changes in response to changes in energy density or, alternatively, from the rate at which temperature changes in response to changes in entropy , which is a measure of disorder in an environment. system.

In heavy ion collisions, entropy can be inferred from the number of electrically charged particles emitted by the collisions. The temperature, on the other hand, can be deduced from the average transverse moment (i.e. the moment transverse to the collision axis) of these particles.

Using data from lead-lead collisions at an energy of 5.02 trillion electronvolts per pair of nucleons (protons or neutrons), the CMS collaboration measured for the first time how temperature varies with entropy in collisions heavy ion centrals, in which the ions collide head-on and overlap almost completely.

From this measurement, they obtained a value for the speed of sound in this medium close to half the speed of light and with record precision: in units of speed of light, the square of the speed of sound is 0.241, with statistical uncertainty. of 0.002 and a systematic uncertainty of 0.016. Using the average transverse moment, they also determined that the effective temperature of the quark-gluon plasma was 219 million electron volts (MeV), with a systematic uncertainty of 8 MeV.

The results match theoretical expectations and confirm that quark-gluon plasma acts as a fluid made up of particles carrying enormous amounts of energy.

The article is published on the arXiv preprint server.