CERN Physicists Catch Gluons in Dense Quantum State Inside Atomic Nuclei

CERN – September 12, 2026

CERN scientists may have caught gluons entering a strange, crowded state that could reshape our understanding of how matter gets its mass.

A CERN experiment has given physicists a much sharper look at how gluons behave inside atomic nuclei, providing new evidence that could help distinguish between two competing explanations of what happens at extremely small scales.

University of Kansas physicist Daniel Tapia Takaki played a leading role in the study, which was conducted as part of the ALICE experiment at CERN's Large Hadron Collider and published in Physical Review Letters.

The researchers report the first multidimensional measurement of incoherent J/ψ (pronounced "JAY-sigh") photonuclear production that tracks both interaction energy and momentum transfer. Together, those measurements allow scientists to examine how gluons are distributed inside atomic nuclei with unprecedented detail.

Why Gluons Matter

Gluons are particles that help bind quarks together through the strong force. Although quarks are commonly described as the fundamental pieces of protons and neutrons, much of the mass of ordinary matter comes from the energy associated with gluons and the strong force.

"Although quarks are often described as the fundamental building blocks of matter, nearly all the mass of the visible universe — from the atoms in our bodies to the matter inside stars — actually comes from the energy carried by gluons and the strong force that binds quarks together," said nuclear physicist Daniel Tapia Takaki, professor of physics & astronomy at KU and member of the ALICE collaboration. "Understanding how gluons behave inside nuclei is therefore essential to understanding how matter itself acquires its mass and structure."

Tapia Takaki helped lead the research while working closely with scientists at the Czech Technical University in Prague. KU has an institutional partnership with the university that includes exchanges involving both students and researchers.

Turning the LHC Into a Gluon Microscope

To examine small variations in the distribution of gluons inside nuclei, the researchers used a technique known as incoherent J/ψ photonuclear production.

"When one of these photons strikes another nucleus, it can briefly produce a particle called the J/ψ, whose production provides a sensitive probe of the underlying gluon structure," Tapia Takaki said. "Many measurements effectively average the gluon distribution across an entire nucleus. Incoherent J/ψ production, by contrast, can reveal local changes in gluon density."

The researchers measured incoherent J/ψ production across photon-nucleus energies ranging from 20 to 633 billion electron volts. They also studied how the process varied with momentum transfer, which determines the spatial scale being examined inside the nucleus.

"Our experiments using incoherent production are like switching from a blurry image to a high-resolution microscope," Tapia Takaki said. "This process allows us to see how gluons fluctuate and organize themselves inside nuclei. 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."

Unexpected Results

The results revealed a striking pattern. At the smallest scales, the ALICE team observed a significant drop in J/ψ production that conventional explanations struggle to account for.

"The results revealed a striking pattern," said Tapia Takaki. "At the smallest scales, we saw a drop in J/ψ production that nuclear shadowing can't explain. This suppression challenges a long-standing explanation known as nuclear shadowing, which has successfully described previous measurements."

In the nuclear shadowing framework, gluons inside a nucleus partially overlap and obscure each other — similar to layers of clouds blocking sunlight — reducing the probability of certain particle production processes.

"This suppression challenges a long-standing explanation known as nuclear shadowing, which has successfully described previous measurements," Tapia Takaki said. "The new measurements indicate that conventional nuclear shadowing alone can't fully explain observed data. Instead, the observations are consistent with a phenomenon called gluon saturation."

Gluon saturation is a theoretical concept in quantum chromodynamics, the physics of the strong force. In this scenario, gluons become so densely packed that they start to interact with each other, limiting how many can exist in a given volume.

What This Means for Nuclear Physics

The findings suggest that gluons may be entering a state of saturation at high densities, a regime that has been predicted by theory but never directly confirmed in experiments.

This matters because understanding gluon behavior at these scales is crucial for advancing our knowledge of the strong force, one of the four fundamental forces of nature. It could also have implications for research into the early universe, when matter existed in a state called quark-gluon plasma.

The ALICE collaboration includes scientists from research institutions worldwide, including KU, the Czech Technical University in Prague, and numerous other European and American universities.

Looking Ahead

The research team plans to conduct further measurements at different energy levels and with more detailed analysis of the momentum transfer data. These studies will help physicists better understand whether gluon saturation is truly occurring and how it might affect our broader understanding of nuclear matter.

This work also highlights the continued importance of the Large Hadron Collider, which was originally built to search for the Higgs boson but has since become a vital tool for exploring many other frontiers in particle physics.

Source: ScienceDaily, September 12, 2026. University of Kansas, September 12, 2026. Physical Review Letters, September 2026.