ALICE measurements suggest that gluons inside nuclei begin behaving collectively at very small scales, favoring gluon saturation over conventional nuclear shadowing alone.
Deep inside atomic nuclei, gluons bind quarks together and help determine the structure of visible matter. A CERN study, with a University of Kansas physicist playing a leading role, has now shown that experiments can distinguish between two competing explanations for how these particles behave in nuclei.
Conducted with the ALICE experiment at CERN’s Large Hadron Collider and published in Physical Review Letters, the research provides the first multidimensional measurement of incoherent J/ψ (pronounced “”JAY-sigh”) photonuclear production across both interaction energy and momentum transfer. The approach gives researchers an unusually detailed picture of how gluons are distributed inside atomic nuclei at high energies.
“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 work in close collaboration with researchers at the Czech Technical University in Prague. KU has an institutional partnership with the university that includes exchanges of researchers and students.
A new view of gluon fluctuations
To examine variations in gluon density at finer spatial scales than earlier studies could reach, the researchers turned to a technique known as incoherent J/ψ photonuclear production.
“The measurements were performed using data collected during Run 2 of the Large Hadron Collider, where fast-moving lead nuclei pass close to one another without directly colliding,” Tapia Takaki said. “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 can briefly produce a particle called the J/ψ, whose production provides a sensitive probe of the underlying gluon structure.”
Rather than averaging the gluon distribution across an entire nucleus, incoherent J/ψ production can reveal local variations in gluon density. That sensitivity lets researchers investigate structures smaller than a proton. Gluon fields exist inside every atomic nucleus and contribute to nearly all the mass of visible matter, yet understanding how they behave collectively remains a major problem in modern physics.
“Our experiments using incoherent production is 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.”
Tapia Takaki compared the scale to examining a nucleus enlarged to the size of a football stadium. At ALICE’s highest resolution, the experiment could distinguish features only a few yards wide on the field.
Evidence points toward gluon saturation
“At these extraordinary scales, we observe evidence that the gluons begin to behave collectively, a phenomenon known as gluon saturation,” he said
Tapia Takaki has helped develop this experimental approach as well as theoretical models in which gluons cluster into localized regions of especially high density, sometimes called “hot spots.” Under the energy-dependent hot-spot model, these dense regions change as collision energy increases and may carry signatures of previously unexplored behavior in the strong interaction.
For the latest analysis, the researchers measured incoherent J/ψ production across photon-nucleus energies ranging from 20 to 633 billion electron volts. At the same time, they tracked changes with momentum transfer, which determines the spatial scale at which the nucleus is being examined.
“The results revealed a striking pattern,” said the KU researcher. “At the smallest spatial scales explored in the experiment, the production rate of J/ψ particles is significantly suppressed, with a statistical significance of about three standard deviations.”
Nuclear shadowing falls short
Tapia Takaki said the suppression poses a challenge to a long-standing explanation known as “nuclear shadowing,” which has successfully accounted for earlier measurements.
“In that 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,” he said.
The latest measurements show that conventional nuclear shadowing by itself cannot fully account for what ALICE observed.
“Instead, the observations are consistent with a different phenomenon known as ‘gluon saturation,’ predicted by the theory of quantum chromodynamics, which describes the strong force,” Tapia Takaki said. “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.”
Reference: “Evidence for J/ψ Suppression in Incoherent Photonuclear Production” by S. Acharya, A. Agarwal, G. Aglieri Rinella, L. Aglietta, M. Agnello, N. Agrawal, Z. Ahammed, S. Ahmad, S. U. Ahn et al. (ALICE Collaboration), 29 July 2026, Physical Review Letters.
DOI: 10.1103/rmwb.75m7
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