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Brookhaven | Long Island, NY | USA

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The Glue that Binds Us All

RHIC, the Relativistic Heavy Ion Collider at Brookhaven Lab, found it first: a “perfect” liquid of strongly interacting quarks and gluons – a quark-gluon plasma (QGP) – produced by slamming heavy ions together at close to the speed of light. The fact that the QGP produced in these particle smashups was a liquid and not the expected gas, and that it flowed like a nearly frictionless fluid, took the physics world by surprise. These findings, now confirmed by heavy-ion experiments at the Large Hadron Collider (LHC) in Europe, have raised compelling new questions about the nature of matter and the strong force that holds the visible universe together.

Similarly, searches for the source of “missing” proton spin at RHIC have opened a deeper mystery: So far, it’s nowhere to be found.

To probe these and other puzzles, nuclear physicists would like to build a new machine: an electron-ion collider (EIC) designed to shine a very bright “light” on both protons and heavy ions to reveal their inner secrets.

“An electron-ion collider would be the brightest, highest-intensity ‘femtoscope’ to shine on the structure of matter,” said Brookhaven theoretical physicist Raju Venugopalan, referring to its ability to discern structures at the scale of femtometers – that’s 10-15 meters, a millionth of a nanometer, or a millionth of a billionth of a meter!

“Snapshots” of matter at that scale over a wide range of energies would offer deeper insight into the substructure of the nucleus, its constituents, and particularly its smallest components, the quarks and gluons and how they interact.

“Increasingly, it’s looking as if gluons and their interactions may hold the keys to many of our puzzles,” Venugopalan said. An electron-ion collider would be the ideal tool for gazing at the “glue” under conditions where scientists believe that it completely dominates the structure of neutrons, protons, and nuclei.

Spin Timeline

Evolution of physicists' understanding of proton spin: from the early view that the spins of the proton's three quarks should make up most if not all the proton's spin (left), to one in which gluons and the motion of quarks and gluons can also play significant roles (center). Current and planned investigations of the angular motion of quarks and gluons — the latter to be carried out at by an electron-ion collider (right) — may help resolve the mystery of the missing source of spin.

If an electron-ion collider becomes a reality, what the physicists learn will offer deeper insight into what holds 99 percent of the matter in the visible universe together. That’s the percentage of everything we see around us – from stars to planets to our own physical forms – that gets its mass from protons and neutrons, and thus ultimately from the quarks and gluons governed by the strong force.

“At the most fundamental level,” Venugopalan said, “we are driven by our curiosity to learn more about what we are made up of.”

Much more about the physics behind an electron-ion collider and the exciting mysteries on the horizon can be found in this feature story at Brookhaven’s website. Those of you interested in time dilation, missing spin, and super-saturated color glass condensate should check it out.

-Karen McNulty Walsh, BNL Media & Communications Office

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