CERN ALICE experiment finds evidence of gluon saturation deep inside atomic nuclei

CERN ALICE experiment finds evidence of gluon saturation deep inside atomic nuclei

A groundbreaking measurement at CERN's Large Hadron Collider has revealed that gluons—the particles that bind quarks together—begin to behave collectively at the smallest spatial scales inside atomic nuclei, providing new evidence for gluon saturation, a phenomenon predicted by quantum chromodynamics.

The study, conducted by the ALICE collaboration and led in part by University of Kansas physicist Daniel Tapia Takaki, was published in Physical Review Letters in September 2026. Researchers achieved unprecedented resolution, probing structures as small as one-quarter the size of a proton.

At the smallest spatial scales examined, the production rate of J/ψ particles—a sensitive probe of gluon structure—was significantly suppressed, with a statistical significance of about three standard deviations. This unexpected pattern challenges the long-standing theory of nuclear shadowing, which has successfully explained earlier measurements.

"Although quarks are often described as the fundamental building blocks of matter, nearly all the mass of the visible universe comes from the energy carried by gluons and the strong force that binds quarks together," Tapia Takaki said. "Understanding how gluons behave inside nuclei is therefore essential to understanding how matter itself acquires its mass and structure."

Turning the LHC into a Gluon Microscope

The measurements were performed using data from Run 2 of the Large Hadron Collider, where fast-moving lead nuclei pass close to one another without directly colliding. In these encounters, intense electromagnetic fields surrounding the nuclei behave like beams of high-energy photons. When one of these photons strikes another nucleus, it briefly produces a J/ψ particle, whose production provides a sensitive probe of the underlying gluon structure.

"Our experiments using incoherent production is like switching from a blurry image to a high-resolution microscope," Tapia Takaki said. "By varying the momentum transfer, our experiment effectively changes the focus of our microscope. At resolutions of 0.6, 0.3 and 0.2 femtometers, ALICE progressively probed smaller regions inside the nucleus. The finest resolution corresponds to structures only about one-quarter the size of a proton."

The new multidimensional measurement tracked both interaction energy (from 20 to 633 billion electron volts) and momentum transfer simultaneously—the first time such an approach has been used to study incoherent J/ψ photonuclear production.

A Challenge to Nuclear Shadowing

Instead of nuclear shadowing, where gluons inside a nucleus partially overlap and obscure each other similar to layers of clouds blocking sunlight, the observations are consistent with gluon saturation. In this regime, gluons become so densely packed that they begin interacting strongly with one another, limiting how many can exist in a given region.

The finding has profound implications for our understanding of quantum chromodynamics, the theory that describes the strong force binding quarks and gluons together. If gluon saturation is confirmed, it would represent a fundamental shift in how physicists understand matter at the smallest scales.

Tapia Takaki has helped pioneer this experimental approach and has contributed to theoretical models in which gluons gather into localized areas of especially high density, sometimes referred to as "hot spots." In the energy-dependent hot-spot model, these dense regions change as collision energy increases. Their behavior could provide signatures of previously unexplored physics involving the strong interaction.

The ALICE collaboration includes researchers from the Czech Technical University in Prague and University of Kansas, which has an institutional partnership with the university involving exchanges of both students and researchers. The work represents years of data analysis and theoretical development to distinguish between competing models of nuclear structure.

As physicists continue to push toward even smaller scales and higher precision, they hope to uncover more signatures of this previously unexplored physics.

Sources:

• University of Kansas, September 12, 2026
• Physical Review Letters, September 2026