Theoretical nuclear physicists have proposed a new method to measure the speed of sound in quark-gluon plasma. Quarks and gluons are basic constituents of nucleons inside nuclei; scientists cannot observe them directly. Physicists create quark-gluon plasma in relativistic heavy ion collisions. These collisions of heavy ions such as gold or lead nuclei cause hundreds of protons and neutrons to smash into each other.
This forms a miniscule fireball in which everything "melts" into quark-gluon plasma. Hadronic particles form when the plasma cools down. The new method uses fluctuations in the numbers of detected particles to reveal the behavior of the speed of sound in heavy-ion collisions. These detected particles, called cumulants, allow the scientists to learn about the heat and energy properties of dense nuclear matter.
The Impact
The speed of sound is determined by the properties of the material through which sound moves. For example, sound travels faster through warm air than cold air. Sound also travels faster in liquids and solids than it does in gases. Measuring the speed of sound therefore helps scientists understand the properties of nuclear matter in extreme conditions, such as those reached in high-energy heavy-ion collisions. These studies reveal the way quarks and gluons interact with each other. They can also inform our understanding of the matter present in the early Universe or the evolution of neutron stars and neutron star mergers.
Summary
In any medium, sound is a wave that propagates by creating regions of alternately compressed and rarefied (less compressed) matter. The speed of sound is related to the magnitude of typical fluctuations in a material's density. Therefore, measuring fluctuations in the density of collections of particles may reveal the behavior of the speed of sound for that collection of particles.
Heavy-ion collision experiments, including the experiments carried out at the Relativistic Heavy Ion Collider, a Department of Energy Office of Science user facility, can measure fluctuations in the number of detected particles. Those measurements can help physicists learn about the speed of sound in dense nuclear matter. These measurements provide information on the arrangement of different forms of matter, from quark-gluon plasmas to neutron stars.
Funding
This work was supported by the Department of Energy Office of Science, Office of Nuclear Physics.