Brookhaven has proposed to upgrade RHIC to 'eRHIC', the world’s first electron-heavy ion collider
The stunning surprise that the early-universe matter created at RHIC behaves more like a liquid than a gas has enriched physicists’ understanding of quantum chromodynamics (QCD) — the theory that describes the interactions of the smallest known components of the atomic nucleus. But it has also raised compelling new questions — questions which have prompted the need for the evolution of RHIC.
Brookhaven Lab proposes to upgrade RHIC in several stages. The first stage, enabling a science program known as RHIC-II, is an ongoing effort to increase RHIC’s collision rate. Later stages will add a new electron beam, known as eRHIC, and a new detector.
RHIC’s collisions of gold-ion beams have been energetic and intense enough to create, in microcosm, the hot, dense conditions scientists believe existed in the first microseconds of the early universe. Predicted to produce a gaseous plasma of liberated quarks and gluons, these collisions instead produced a nearly “perfect” liquid.
RHIC-II, a ten-fold increase in RHIC’s luminosity, or collision rate, will enhance scientists’ ability to study detailed properties of this perfect liquid.
The RHIC-II luminosity improvement will be accomplished thanks to a technological breakthrough at RHIC that allows information about beam imperfections gathered at one location to beat the light-speed beam to a second location, where the imperfections can be corrected.
When protons or nuclei are viewed at very high energies -- as the ions speeding toward collisions at RHIC “see” each other -- their structure is completely dominated by a teeming sea of gluons, the elusive particles exchanged between quarks when they exert forces on one another. Gluons are in some ways like photons, the basic quantum particles of light, but unlike photons, gluons have a propensity for splitting to create additional gluons or recombining to eliminate gluons. At the enormous densities revealed in the high-energy view, it is expected that the splitting and recombination processes reach a balance, creating a force field with universal properties at the heart of all matter. In order to study the so-far poorly probed behavior of this super-strong force field – called a “color glass condensate” – Brookhaven has proposed to turn RHIC into eRHIC, the world’s first electron-heavy ion collider.
Electron scattering provides the best microscope to unveil the distribution of gluons, a counter-intuitive fact since electrons interact directly only with electrically charged particles, while the gluons are neutral. When the electrons collide with heavy nuclei, they can probe the color glass condensate regime at much lower energies than would be required for electron-proton collisions. Thus, eRHIC would be the world’s foremost facility for the study of gluon-dominated matter, illuminating the conditions just before the RHIC collisions that create the nearly "perfect" liquid.
Constructing eRHIC requires technological advances in the acceleration of high-current electron beams, advances being pursued in ongoing research and development at RHIC. By polarizing the electron beams (arranging the spins of the electrons to point in a preferred direction), and allowing them to scatter off RHIC’s polarized proton beams, the study of the proton’s substructure can be greatly advanced. The ongoing RHIC search for the proton’s “missing” spin (the roughly 70% unaccounted for by the orientation of quark and antiquark spins within the proton) can be extended to the contributions from the dominant gluons in the color glass condensate region.