How does a quark-gluon-plasma fireball explode into hadrons?

Quark-gluon plasma (QGP) is an exciting state of matter created by scientists colliding two heavy nuclei in the laboratory. These collisions produce QGP fireballs. The fireball expands and cools according to the laws of fluid dynamics, which govern how fluids behave under various conditions. Eventually, subatomic particles (protons, mesons, and other hadrons, or particles composed of two or more quarks) emerge and are observed and counted by detectors surrounding the collision.

Fluctuations in the number of these particles during each collision carry important information about 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 fluid dynamics of the QGP fireball.

The method is described in the journal Physical Review Letters.

As the QGP fireball expands and cools, it eventually becomes too thin to be described by fluid dynamics. At this stage, QGP has “hardened”. This means that its energy and other quantum properties are carried by hadrons. These are subatomic particles made of quarks, such as protons, neutrons, and mesons. Hadrons “freeze” they freeze information about the final hydrodynamic state of the QGP fireball, allowing particles flowing out of the collision to carry this information to detectors in the experiment.

This study provides a tool to use simulations to calculate observable fluctuations in QGP. This allowed researchers from the University of Illinois at Chicago to use freezing to identify signs of the tipping point between the QGP’s fireball and gaseous hadronized states. This tipping point is one of scientists’ unsolved questions about quantum chromodynamics, the theory of strong gluon-driven interactions between quarks.

Fluctuations in QGP carry information about the regions of the QCD phase diagram that are “frozen” by collisions. This makes linking hydrodynamic fluctuations to observed hadron fluctuations a critical step in converting experimental measurements into QCD phase diagrams. Large event-by-event fluctuations are a distinct experimental signature of tipping points.

Data from the Run-I Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC) hint at the existence of a critical point. To follow this lead, the researchers proposed a novel and general method to convert hydrodynamic fluctuations into hadronic multiplicity fluctuations.

This approach neatly overcomes the challenges faced by previous attempts to solve this problem. Crucially, the new method, based on the principle of maximum entropy, retains all information about fluctuations in conserved quantities described by fluid dynamics. This novel freezing procedure will find application in theoretical calculations of event-by-event fluctuations and correlations observed in experiments such as RHIC’s beam energy scanning procedure designed to map QCD phase diagrams.