A New Area of Physics
RHIC created a new state of hot, dense matter out of the quarks and gluons that are the basic particles of atomic nuclei, but it turned out to be 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 behaved more like a liquid.
Gluons and quarks
Ions about to collide
Just after collision
The "perfect" liquid
A "Perfect" Liquid
RHIC scientists had expected collisions between two beams of gold nuclei to mimic conditions of the early universe and produce a gaseous plasma of the smallest components of matter — the quarks and gluons that make up ordinary protons and neutrons. But instead of behaving like a gas, the early-universe matter created in RHIC’s energetic gold-gold collisions behaved more like a liquid.
And it’s not just any liquid, but one with coordinated collective motion, or “flow,” among the constituent particles. Scientists describe this fluid motion as nearly perfect because it can be explained by the equations of hydrodynamics for a fluid with virtually no viscosity, or frictional resistance to flow. In fact, the high degree of collective interaction and rapid distribution of thermal energy among the particles, as well as the extremely low viscosity in the matter formed at RHIC, make it the most nearly perfect liquid ever observed.
Quark-Gluon Plasma
RHIC’s perfect liquid was also the hottest matter ever created in a laboratory at the time, measuring some 4 trillion degrees Celsius — about 250,000 times hotter than the center of the Sun. That’s far above the temperature at which protons and neutrons “melt” to free their constituent quarks and gluons, showing definitively that RHIC’s perfect liquid was hot enough to be the long-sought quark-gluon plasma.
Since these discoveries, RHIC physicists have made precision measurements of the quark-gluon plasma, including its temperature at different stages, how it swirls — it’s the swirliest matter ever! — how quarks and gluons in the primordial soup transition under various conditions of temperature and pressure to the nuclear matter that makes up atoms in our world today, and how collisions of even small particles can create tiny drops of quark-gluon plasma.
Evidence of Gluons' Contribution to Spin
In the proton spin program, RHIC’s measurements greatly improved the precision with which scientists could determine gluons’ contribution to proton spin, along with the contribution from quarks. This effort was motivated by surprising results from experiments elsewhere in the 1980s showing that quarks contribute only a fraction to this quantum property. Gluons were initially assumed to contribute the rest. RHIC’s measurements revealed that gluons contribute about as much as the quarks. At least some of the gluons are spin-aligned with the spin of the proton they are in. But there is still more to explore in this “spin puzzle,” which will be a major research focus at the future Electron-Ion Collider (EIC).
Exotic Findings and Interesting Connections
Throughout 25 years of operations, RHIC’s data has yielded other unusual discoveries and interesting connections. One was a link with string theory and ultra-cold atoms at the opposite end of the temperature scale from the superhot quark-gluon plasma. Another was an observation that “bubbles” formed within the hot plasma appeared to disobey fundamental symmetries.
RHIC collisions also produced the heaviest antimatter nucleus yet discovered, and the first containing an anti-strange quark. This discovery of exotic antimatter extended physicists’ “map” of nuclei into a new frontier, offering insights into neutron stars and fundamental asymmetries in the early universe. RHIC collisions also showed how matter could be created from particles of light surrounding speeding gold ions and even from the “nothingness” of empty space.
