LAWRENCE — A University of Kansas physicist played a leading role in a CERN study showing that two rival explanations for how gluons behave inside atomic nuclei can now be experimentally distinguished.
The research, carried out within the ALICE experiment at CERN's Large Hadron Collider and published in Physical Review Letters , reports the first multidimensional measurement of incoherent J/ψ (pronounced ""JAY-sigh") photonuclear production as a function of both interaction energy and momentum transfer. The measurement gives scientists their clearest view yet of how gluons, the particles that bind quarks together, are arranged 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 played a lead role in the research, collaborating closely with investigators at the Czech Technical University in Prague, where KU maintains an institutional partnership that includes exchanges of students and researchers.
To study how gluons fluctuate within nuclei with more spatial resolution than previously possible, the team used an experimental technique called 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."
Unlike other measurements that average over the entire nucleus, incoherent J/ψ production is sensitive to local fluctuations in gluon density, allowing researchers to probe structures smaller than a proton. These intense gluon fields, present inside every atomic nucleus, make up nearly all the visible matter in the universe. Yet their collective behavior remains one of the greatest challenges 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."
According to Tapia Takaki, if a nucleus was the size of a football stadium, the ALICE collaboration's highest-resolution measurement would be capable of distinguishing details only a few yards across on the playing field.
"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 been a pioneer of this approach and contributed to theoretical models describing how gluons form localized regions of high density — sometimes called "hot spots." In the energy-dependent hot-spot model, these regions evolve with collision energy and can provide signatures of new physics in the strong interaction.
In the new study, researchers measured incoherent J/ψ production across a wide range of photon-nucleus energies — from 20 to 633 billion electron volts — while also examining how the interaction changes with momentum transfer, which corresponds to how finely the nucleus is being probed.
"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."
He said this suppression challenges a long-standing explanation known as "nuclear shadowing," which has successfully described previous 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 new measurements indicate that conventional nuclear shadowing alone can't fully explain observed data.
"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."
