The Physics of RHIC
For 25 years, physicists from around the world have used the Relativistic Heavy Ion Collider (RHIC) to explore some of Nature's most basic — and intriguing — ingredients and phenomena. Though RHIC's collisions have ended, analysis of its data continues. Here's a look at the major physics areas explored at RHIC.
Heavy Ion Collisions
RHIC was the first machine in the world capable of colliding heavy ions — the nuclei of heavy atoms such as gold stripped of their outer cloud of electrons. RHIC often used gold, one of the heaviest common elements, because its nucleus is densely packed with particles. RHIC accelerated two beams of gold ions to nearly the speed of light (what physicists call relativistic speeds). The beams traveled in opposite directions around RHIC's 2.4-mile, two-lane ring-shaped “racetrack.” At six intersections where the lanes crossed, ions would collide head on. When conditions were right, these collisions “melted” the protons and neutrons, liberating for a brief instant their innermost building blocks — quarks and gluons. These collisions essentially recreated a unique form of matter from the very early universe known as a quark-gluon plasma. Just after the collision, thousands of new particles would form as the quark-gluon plasma cooled off. Each of these particles striking one of RHIC's detectors provided clues about what happened in the collision zone. Physicists continue to sift through those clues to make discoveries about the quark-gluon plasma.
1. Ions about to collide*
2. Ion collision
3. Quarks, gluons freed
4. Quark-gluon plasma created
*The ions appear as flat disks because of a relativistic effect called “Lorentz contraction.”
About Quarks & Gluons
Scientists believe that all protons and neutrons — the building blocks of atomic nuclei — are made up of quarks and the gluons that bind them together with the strongest force in nature.
Theory holds that for a brief time at the beginning of the universe there were no protons and neutrons, only free quarks and gluons. However, as the universe expanded and cooled, the quarks and gluons bound together and have remained virtually inseparable for nearly 14 billion years. RHIC was the first instrument humans built that could take us “back in time” to see how matter behaved at the start of the universe.
Physicists around the world have studied data from RHIC collisions. What they’ve learned at RHIC can be applied in nuclear physics (the study of atomic nuclei), particle physics (the study of the atom’s constituents), astrophysics (the study of stars and planets), condensed matter physics (the science of solid matter), and cosmology (the study of the universe).
Hotter than the Sun
When RHIC was running, collisions occurred thousands of times per second. Each collision acted like a microscopic pressure cooker, producing temperatures and pressures more extreme than exist now even in the hottest stars. Experiments at RHIC revealed the temperature inside a collision could be 250,000 times hotter than the center of the sun.
But the size of the ions that created these superhot fireballs is tiny. As a result, the impact of the speeding ions on each other was about the same as the impact of a mosquito hitting a screen door on a summer evening. And each speck of hot matter created in RHIC collisions lasted only a few billionths of a second — too small and too brief to be dangerous — but still super interesting to physicists!
Spin Physics
Protons are made of quarks and are held together by emitting and absorbing gluons. All of these particles have an intrinsic property known as spin. Physicists have long thought that the spin of a proton was simply the sum of the spins of its three main component quarks, known as valence quarks. But experiments have shown that quarks account for less than a third of the proton’s spin.
What accounts for the rest? RHIC was the world’s only machine capable of colliding high-energy beams of polarized protons to investigate this question.
RHIC’s two concentric rings were made up of 1,740 superconducting magnets, mounted end-to-end.
Counter-rotating particle beams could cross at six intersections around the RHIC ring. Detectors were originally located at four intersection points.
A three-dimensional view of the calculated particle paths resulting from collisions occurring within RHIC's STAR detector.
Black Holes?
Before RHIC began operations in 2000, some people were unnecessarily concerned that it would produce black holes that would threaten Earth.
Here's why those concerns were unfounded.
