A review article appearing in the July 20, 2012, issue of the journal Science describes groundbreaking discoveries that have emerged from the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory, synergies with the heavy-ion program at the Large Hadron Collider (LHC) in Europe, and the compelling questions that will drive this research forward on both sides of the Atlantic. With details that help enlighten our understanding of the hot nuclear matter that permeated the early universe, the article is a prelude to the latest findings scientists from both facilities will present at the next gathering of physicists dedicated to this research — Quark Matter 2012, August 12-18 in Washington, D.C.
“Nuclear matter in today’s universe hides inside atomic nuclei and neutron stars,” begin the authors, Barbara Jacak, a physics professor at Stony Brook University and spokesperson for the PHENIX experiment at RHIC, and Berndt Mueller, a theoretical physicist at Duke University. Collisions between heavy ions at machines like RHIC, running since 2000, and more recently, the LHC, make this hidden realm accessible by recreating the extreme conditions of the early universe on a microscopic scale. The temperatures achieved in these collisions — more than 4 trillion degrees Celsius, the hottest ever created in a laboratory — briefly liberate the subatomic quarks and gluons that make up protons and neutrons of ordinary atomic nuclei so scientists can study their properties and interactions.
“Quarks and the gluons that hold them together are the building blocks of all the visible matter that exists in the universe today — from stars, to planets, to people,” Jacak said. “Understanding the evolution of our universe thus requires knowledge of the structure and dynamics of these particles in their purest form, a primordial ‘soup’ known as quark-gluon plasma (QGP).”
RHIC was the first machine to demonstrate the formation of quark-gluon plasma, and determine its unexpected properties. Instead of an ideal gas of weakly interacting quarks and gluons, the QGP discovered at RHIC behaves like a nearly frictionless liquid. This matter’s extremely low viscosity (near the lowest theoretically possible), its ability to stop energetic particle jets in their tracks, and its very rapid attainment of such a high equilibrium temperature all suggest that the fluid’s constituents are quite strongly interacting, or coupled.
“Understanding strongly coupled or strongly correlated systems is at the intellectual forefront of multiple subfields of physics,” the authors write. The findings at RHIC have unanticipated connections to several of these, including conventional plasmas, superconductors, and even some atoms at the opposite extreme of the temperature scale — a minute fraction of a degree above absolute zero — which also behave as a nearly perfect fluid with vanishingly low viscosity when confined within an atomic trap.