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The unique and powerful tools of the Electron-Ion Collider will cast fresh light on the forces that bind protons and neutrons together to form nuclei

How do quarks and gluons make up nearly all of the visible matter in the universe?

Nearly all visible matter—everything made of atoms—is made up of point-like quarks and gluons that inhabit a quantum world with remarkable features that defy simple intuition. For instance, quarks are nearly massless, and gluons are massless, but their interactions are sufficiently strong to generate the energy that gives mass to matter through Einstein's iconic equation: E=mc2. Another peculiar feature: These building blocks of visible matter are themselves not visible! Quarks and gluons, unlike electrons or photons of light, can never be observed directly. But they do carry a form of charge, whimsically called “color.”

The force generated by color charges, which binds quarks and gluons together, is the strongest in nature. And the nature of the quantum vacuum is inextricably tied up with the strong force generated by quark and gluon charges. But exactly how color-charged quarks and gluons form colorless protons and neutrons—and how tiny residuals of color forces bind those protons and neutrons together to form nuclei—remain among the most profound mysteries in science. The unique and powerful tools of the Electron-Ion Collider will cast fresh light on these questions.

Precision 3D imaging of protons and nuclei

The Electron-Ion Collider will take three-dimensional precision snapshots of the internal structure of protons and atomic nuclei. As they pierce through the larger particles, the high-energy electrons will interact with the internal microcosm to reveal unprecedented details—zooming in beyond the simplistic structure of three valence quarks bound by a mysterious force. Recent experiments indicate that the gluons—which carry the strong force—multiply and appear to linger within particles accelerated close to the speed of light, and play a significant role in establishing key properties of protons and nuclear matter. By taking images at a range of energies, an EIC will reveal features of this “ocean” of gluons and the “sea” of quark-antiquark pairs that form when gluons interact—allowing scientists to map out the particles’ distribution and movement within protons and nuclei, similar to the way medical imaging technologies construct 3D dynamic images of the brain. These studies may help reveal how the energy of the massless gluons is transformed through E=mc2 to generate most of the mass of the visible universe.

Solving the proton spin puzzle

The Electron-Ion Collider will be the world’s first polarized electron-proton collider where both the electron and proton beams have their spins aligned in a controllable way. This polarization makes it possible to make precision measurements of how a proton’s constituent quarks and gluons and their interactions contribute to the proton’s intrinsic angular momentum, or spin. Spin influences the proton’s optical, electrical, and magnetic characteristics and makes technologies such as MRI scanning work, but its origin has eluded physicists ever since experiments in the 1980s revealed that quarks can account for only about a third of the total spin. More recent experiments show that gluons make a significant contribution, perhaps even more than the quarks. The Electron-Ion Collider will produce definitive measurements of the quark and gluon contributions to spin, including how these particles' movements within the proton microcosm affect its overall spin structure—thus providing the final pieces needed to solve this longstanding puzzle.

Search for saturation

Capturing the dynamic action of gluons within protons and nuclei will give scientists a way to test their understanding of these particles’ ephemeral properties. As gluons flit in and out of the vacuum, multiplying and recombining, scientists suspect they may reach a steady state of saturation called a “color glass condensate.” This unique form of nuclear matter gets its name from the “color” charges that mediate the interactions of the strong nuclear force, and the dense, glasslike walls these particles are thought to form in nuclei accelerated to nearly the speed of light, seemingly suspended by the effects of time dilation. Scientists will use the Electron-Ion Collider to search for definitive proof of whether this form of matter exists, and test the limits of gluons’ ability to expand beyond the bounds of a single proton/neutron inside a nucleus. They’ll also explore the mechanism that keeps gluon growth in check, like a lid clamping down on an overflowing popcorn pot. Precisely measuring the strength of the gluon fields, which constitute the strongest fields in nature, will tell us how gluons interact with each other and how they contribute to building the bulk of visible matter in the universe today.

Quark and gluon confinement

The very energetic interactions of the EIC’s electron beams with protons and nuclei will deliver sufficiently strong kicks to release quarks and gluons from their confinement within these composite particles. The liberated color-charged quarks and gluons will then quickly pick up color charge from the quantum vacuum to reconstitute themselves as colorless matter. The EIC’s high energy and intensity will thus allow unprecedented studies of the interplay of quarks and gluons with the vacuum. In particular, it can address how these interactions are modified if the quarks and gluon spins are polarized or exist alongside other color charges as they would be in a large nucleus. These studies may uncover yet to be discovered fundamental features of the quantum vacuum that help explain why color-charged quarks and gluons remain confined.

Quarks and gluons in nuclei

While past experiments have shown that the distribution of quarks in a nucleus is different from their distribution in a proton (an effect called nuclear shadowing), we do not know whether the distribution of gluons is similarly modified in nuclei. Another recently discovered puzzle is that, contrary to predictions, both light and heavy quarks passing through a quark-gluon plasma appear to lose identical amounts of energy. Electron-nucleus collisions at the EIC, with its wide range of nuclei at high energy, will be the ideal tool to study the distribution of quarks and gluons in nuclei and to learn how light and heavy quarks interact and lose energy in nuclear matter.

Electron-Ion Collider User Group

An international affiliation of scientists dedicated to developing and promoting the scientific, technological, and educational goals and motivations for an EIC.

Center for Frontiers in Nuclear Science

An international research hub for physics related to the EIC. It brings together Stony Brook University faculty, Brookhaven staff, and scientists around the world with students and new scientific talent to investigate the structure of nucleons and nuclei.


The Electron-Ion Collider Center at Jefferson Lab (EIC2@JLab) is an organization to advance and promote the science program at the EIC.

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Brochure (PDF)

Brookhaven National Lab's EIC Directorate coordinates with domestic and international partners to deliver the EIC construction project.

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Brookhaven National Laboratory advances fundamental research in nuclear and particle physics to gain a deeper understanding of matter, energy, space, and time; applies photon sciences and nanomaterials research to energy challenges of critical importance to the nation; and performs cross-disciplinary research on climate change, sustainable energy, and Earth’s ecosystems.