A "Perfect" Liquid at RHIC
Evidence to date suggests that RHIC’s gold-gold collisions are indeed creating a new state of hot, dense matter, but one quite different and even more remarkable than had been predicted. Instead of behaving like a gas of free quarks and gluons, as was expected, the matter created in RHIC’s heavy ion collisions appears to be more like a liquid.
That evidence comes from measurements by the four RHIC detectors of unexpected patterns in the trajectories taken by the thousands of particles produced in individual collisions. Instead of dissipating randomly, as would be expected in a gas, the particles tend to move collectively in response to variations of pressure across the volume formed by the colliding nuclei. Scientists refer to this phenomenon as “flow,” since it is analogous to the properties of fluid motion.
However, unlike ordinary liquids, in which individual molecules move about randomly, the hot matter formed at RHIC seems to move in a pattern that exhibits a high degree of coordination among the particles — somewhat like a school of fish that responds as one entity while moving through a changing environment. The scientists describe this as fluid motion that is nearly “perfect,” because it can be explained by equations of hydrodynamics. It fact, the high degree of collective interaction and rapid distribution of thermal energy among the particles, as well as the extremely low viscosity (or resistance to flow) in the matter being formed at RHIC, make it the most nearly perfect liquid ever observed.
Planned upgrades to RHIC will expand the facility’s reach for understanding this liquid phase of the early universe and allow researchers to investigate entirely new but complementary areas of physics.
The first upgrade, known as RHIC-II, will increase the collider’s luminosity, or collision rate approximately 10-fold, and improve the sensitivity of the big detectors to record extremely rare processes — some of which occur in fewer than one in a billion collisions — to reveal detailed characteristics of the new form of matter.
Another upgrade, known as eRHIC, would add a high-energy electron ring to create the world’s only electron-heavy ion collider. By colliding heavy nuclei with electrons at energies never before achieved in the laboratory, physicists expect to probe another new form of matter known as a color glass condensate — hypothesized to be the maximum density state that can be achieved by particles subject to the strong force.
These upgrades exemplify RHIC’s flexibility to explore the newest and most intriguing and fundamental questions about the substructure of matter.