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

Neutron stars in the universe, ultracold atomic gases in the laboratory, and quark–gluon plasma produced by the collision of nuclear nuclei at the Large Hadron Collider (LHC): these may seem unrelated, but surprisingly, they have something in common. Is. They are all fluid-like states of matter made up of strongly interacting particles. Insights into the properties and behavior of any of these nearly perfect fluids may be key to understanding nature at scales that are widely separated.

In a new paper, the CMS collaboration reports the most accurate measurements in history of the speed at which sound travels in a quark–gluon plasma, offering new insights into this super-hot state of matter.

Sound is a longitudinal wave that travels through a medium, causing compression and rarefaction of matter in the direction of its motion. The speed of sound depends on the properties of the medium, such as its density and viscosity. Therefore, it can be used as a probe of the medium.

At the LHC, quark–gluon are created in collisions between plasmas. Heavy ions. In these collisions, for a fraction of a second, an enormous amount of energy is packed into a volume at most the size of an atomic nucleus. Quarks and gluons emitted from Clash move freely within this region, providing 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 it changes in response to changes in pressure. Energy density or, alternatively, by the rate at which entropy changes in response to changes in temperature, a measure of disorder in a system.

In heavy ion collisions, the entropy can be estimated from the number of electrically charged particles emitted by the collision. On the other hand, the temperature can be derived from the average transverse momentum (ie, the momentum transverse to the collision axis) of these particles.

Using data from lead-lead collisions at energies of 5.02 trillion electron volts per nucleon (proton or neutron), the CMS collaboration has measured for the first time how temperature varies with entropy in a central heavy ion collision. , in which the ions collide and almost completely overlap.

From this measurement, they obtained a value for the speed of sound in this medium that is about half the speed of light, with record accuracy: Speed ​​of lightSquare Speed ​​of sound is 0.241, with a statistical uncertainty of 0.002 and a systematic uncertainty of 0.016. Using the average transverse momentum, they also determined the effective temperature of the quark–gluon plasma to be 219 million electron volts (MeV), with a systematic uncertainty of 8 MeV.

The results match the theoretical expectation and confirm that A quark-Glon plasma acts as a fluid made of particles that carry a lot of energy.

There is paper. published On arXiv Preprint server