How do quark-gluon-plasma fireballs explode into hadrons?

Quark-gluon plasma (QGP) is an exciting state of matter that scientists create in the laboratory through the collision of two heavy nuclei. These collisions produce a QGP fireball. The fireball expands and cools according to the laws of hydrodynamics, which govern the behavior of fluids under various conditions. Eventually, subatomic particles (protons, pions and other hadrons, or particles composed of two or more quarks) emerge and are observed and counted by the detectors surrounding the collision.

Fluctuations in the number of these particles from one collision to the next contain important information about the QGP. However, extracting this information from what scientists can observe is a difficult task. An approach called the maximum entropy principle provides a crucial link between these experimental observations and the hydrodynamics of the QGP fireball.

The approach is described in the review Physical Examination Letters.

As a QGP fireball expands and cools, it eventually becomes too dilute to be described by hydrodynamics. At this stage, the QGP has “hadronized”. This means that its energy and other quantum properties are carried by hadrons. These are subatomic particles such as protons, neutrons and pions made of quarks. The hadrons “freeze”: they freeze information about the final hydrodynamic state of the QGP fireball, allowing the particles resulting from the collision to transmit this information to the detectors during an experiment.

The research provides a tool to use simulations to calculate observable QGP fluctuations. This allowed researchers at the University of Illinois at Chicago to use the gel to identify evidence of a critical point between a QGP fireball and a gaseous hadronized state. This critical point is one of scientists’ unresolved questions about quantum chromodynamics, the theory of strong interactions between quarks induced by gluons.

QGP fluctuations contain information about the region of the QCD phase diagram where collisions “freeze”. This makes connecting fluctuations in hydrodynamics to fluctuations in observed hadrons a crucial step in translating experimental measurements into the QCD phase diagram map. Large event-by-event fluctuations are experimental signatures revealing the critical point.

Data from the Relativistic Heavy-Ion Collider (RHIC) Run-I Beam Energy Scan (BES) program suggests the presence of the critical point. To follow this idea, researchers proposed a new and universal approach to convert hydrodynamic fluctuations into fluctuations of hadronic multiplicities.

This approach elegantly overcomes the challenges encountered in previous attempts to solve this problem. Above all, the new approach based on the principle of maximum entropy preserves all the information on the fluctuations of conserved quantities described by hydrodynamics. The new freezing procedure will find applications in theoretical calculations of fluctuations and event-by-event correlations observed in experiments such as RHIC’s Beam Energy Scan program aimed at mapping the QCD phase diagram.