Particle Puzzle: How Do Quark-Gluon-Plasma Fireballs Explode Into Hadrons?

Deep inside particle accelerators, scientists are recreating a state of matter that existed just microseconds after the Big Bang—quark gluon plasma. This ultra-hot, ultra-dense “soup” of quarks and gluons helps researchers unlock the secrets of how the universe’s building blocks first formed.

Introduction to Quark Gluon Plasma (QGP)

To study quark gluon plasma (QGP), scientists smash heavy atomic nuclei together at nearly the speed of light. The result? A fiery mini-explosion that creates a QGP “fireball.” As it rapidly expands and cools, the plasma transforms into familiar particles like protons, pions, and other hadrons—particles made of two or more quarks.

These newly formed particles are caught and counted by powerful detectors, and their numbers offer crucial clues about the short-lived QGP. But there’s a challenge: the information is hidden in complex patterns of particle fluctuations from one collision to the next.

That’s where a clever method known as the maximum entropy principle comes in. It helps researchers decode these fluctuations, linking what they observe in the lab to the behavior of the QGP fireball as it cools and transitions into ordinary matter.

Hadronization and New Research

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

New research provides a tool for using simulations to compute observable fluctuations in the QGP. This allowed the researchers to use freeze out to identify hints 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 the strong gluon-driven interactions between quarks.

Experimental Implications and the Maximum Entropy Principle

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

Data from the Run-I Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC) hint at the presence of the critical point. To follow this hint, researchers at the University of Illinois, Chicago, proposed a novel and universal approach to converting hydrodynamic fluctuations into fluctuations of hadron multiplicities.

The approach elegantly overcomes challenges faced by previous attempts to solve this problem. Crucially, the new approach based on the maximum-entropy principle preserves all the information about the fluctuations of conserved quantities described by hydrodynamics.

The novel freeze-out procedure will find applications in the theoretical calculations of event-by-event fluctuations and correlations observed in experiments such as the Beam Energy Scan program at RHIC aimed at mapping the QCD phase diagram.

Reference: “Maximum Entropy Freeze-Out of Hydrodynamic Fluctuations” by Maneesha Sushama Pradeep and Mikhail Stephanov, 20 April 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.162301

This work is supported by the Department of Energy Office of Science, Office of Nuclear Physics within the framework of the BEST Topical Collaboration.

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