Heavy-Ion Collisions

Introduction: The Quark-Gluon-Plasma

It is believed that shortly after the creation of the universe in the Big Bang all matter was susceptible to one of the four basic forces of nature, Quantum Chromo-Dynamics (QCD), and was in a state called the Quark Gluon Plasma (QGP). Due to the rapid expansion of the Universe, this plasma went through a phase transition to form hadrons – most importantly nucleons – which constitute the building blocks of ordinary matter as we know it today. The investigation of QGP properties will yield important novel insights into the development of the early universe and the behavior of QCD under extreme conditions.

Ultra-relativistic Heavy-Ion Collisions

Ultrarelativistic heavy-ion collisions offer the unique ability to investigate this hot and dense QCD matter under laboratory conditions and to probe the properties of the QGP. However, due to the fundamental confining properties of the physical QCD vacuum, the deconfined quanta (i.e. the quarks and gluons) are not directly observable. What is observable are hadronic and leptonic residues of this transient deconfined state. Currently, experiments are underway at the  Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) and the Large Hadron Collider (LHC) at the European Center for Nuclear & Particle Physics Research (CERN), to create a QGP and investigate its properties. The connection between the measurements made by the experiments at RHIC and LHC and the properties of the QGP state have to be made via a detailed comparison of data to computational models of the QGP formation and evolution.

Two views of one of the first full-energy collisions between gold ions at Brookhaven Lab’s Relativistic Heavy Ion Collider, as captured by the¬†Solenoidal Tracker At RHIC (STAR)¬†detector. The tracks indicate the paths taken by thousands of subatomic particles produced in the collisions as they pass through the STAR Time Projection Chamber, a large, 3-D digitial camera.

Transport Models

A promising approach to connect the transient QGP state with the experimentally observable hadronic final state is the application of transport theory. Transport theory offers the possibility to cast the entire time evolution of the heavy-ion reaction – from its initial state to freeze-out – into one consistent framework.

Cartoon of a Ultra-relativistic heavy-ion collision. Left to right: the two nuclei approach, collide, form a QGP, the QGP expands and hadronizes, finally hadrons rescatter and freeze out

Current state-of-the-art transport models employ relativistic viscous hydrodynamics for the early dense reaction phase and microscopic non-equilibrium dynamics for the later, dilute reaction stages.