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.
“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.”
Many measurements effectively average the gluon distribution across an entire nucleus. Incoherent J/ψ production, by contrast, can reveal local changes in gluon density. That makes it possible to investigate structures even smaller than a proton.
The powerful gluon fields inside atomic nuclei are central to the structure of nearly all visible matter, but physicists still do not fully understand how large numbers of gluons behave together.
“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 offered another way to picture that level of precision. If an atomic nucleus were enlarged to the size of a football stadium, the experiment’s highest resolution would be fine enough to distinguish features only a few yards wide on the field.
“At these extraordinary scales, we observe evidence that the gluons begin to behave collectively, a phenomenon known as gluon saturation,” he said
Gluon Hot Spots Come Into Focus
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 scientists with signatures of previously unexplored physics involving the strong interaction.
For the new work, 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.
“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.”
A Challenge to Nuclear Shadowing
That unexpected suppression presents a challenge for a long-standing explanation called “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 suggest that conventional nuclear shadowing by itself is not enough to account for the observed pattern.
Instead, the results are consistent with gluon saturation, a phenomenon predicted by quantum chromodynamics, the theory that describes the strong force.
“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.”
