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Critical clues in quark fireball freeze-out

Scientists delve into the intriguing realm of quark-gluon plasma (QGP) by orchestrating collisions between heavy in laboratories. The aftermath of these collisions births a QGP fireball, whose expansion and cooling adhere to hydrodynamics, the governing principles of fluid behavior in diverse conditions. Detectors encircling the collision meticulously observe and count emerging subatomic particles like protons, pions, and other hadrons, composed of two or more quarks.

Navigating the complexities of QGP involves deciphering fluctuations in numbers across collisions, providing vital insights. The maximum entropy principle emerges as a linchpin, bridging experimental observations and the hydrodynamics of the QGP fireball. As the QGP fireball expands and cools, it undergoes “hadronization,” wherein energy and quantum properties transition to hadrons—particles like protons and neutrons made up of quarks. These hadrons freeze out, preserving information about the QGP's final hydrodynamic state and carrying it to detectors.

Researchers from the University of Illinois, Chicago, harness simulations to compute observable fluctuations in the QGP, utilizing freeze-out to unveil hints of a critical point between the QGP fireball and a gaseous hadronized state. This critical point remains an enigma within quantum chromodynamics, the theory governing strong gluon-driven interactions between quarks.

Fluctuations in the QGP provide a window into the region of the Quantum Chromodynamics (QCD) phase diagram where collisions freeze out. Connecting these fluctuations to hydrodynamics becomes pivotal for translating experimental measurements into a map of the QCD phase diagram. Notably, substantial event-by-event fluctuations serve as experimental signatures of the critical point.

Insights from the Run-I Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC) hint at the elusive critical point. To pursue this lead, researchers propose a novel, universal approach converting hydrodynamic fluctuations into hadron multiplicity fluctuations. This approach, grounded in the maximum entropy principle, elegantly surmounts challenges faced by earlier endeavors, preserving crucial information about fluctuations in conserved quantities described by hydrodynamics.

The innovative freeze-out procedure holds promise for theoretical calculations of event-by-event fluctuations and correlations observed in experiments like the Beam Energy Scan program at RHIC, contributing to the ongoing effort to map the QCD phase diagram.

Source: US Department of Energy

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