An international team of physicists working with the ALICE experiment at CERN’s Large Hadron Collider has provided the scientific community with a dramatically sharper look at the behavior of gluons inside atomic nuclei. The new findings offer compelling evidence that could help researchers distinguish between two competing theoretical explanations of what occurs at extremely small subatomic scales, bringing humanity one step closer to understanding how ordinary matter acquires its mass and structural integrity.
University of Kansas physicist Daniel Tapia Takaki played a leading role in the study, which was conducted as part of the massive ALICE collaboration and published in the journal Physical Review Letters. The research marks a significant milestone in nuclear physics, detailing the first multidimensional measurement of incoherent J/ψ (pronounced "JAY-sigh") photonuclear production that simultaneously tracks both interaction energy and momentum transfer. Together, these advanced measurements allow scientists to map how gluons are distributed inside atomic nuclei with a level of detail that was previously unattainable.
To appreciate the significance of the breakthrough, researchers emphasize the fundamental role that gluons play in the universe. While quarks are commonly described as the primary building blocks of protons and neutrons, they only account for a fraction of the mass found in ordinary matter. The vast majority of the mass of the visible universe—stretching from the atoms that compose the human body to the dense matter churning inside stars—actually originates from the immense energy carried by gluons and the strong force that binds quarks together.
"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 Daniel Tapia Takaki, professor of physics and astronomy at the University of Kansas and a dedicated 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 rigorous research effort while collaborating closely with scientists at the Czech Technical University in Prague. The University of Kansas maintains an active institutional partnership with the Czech institution, facilitating ongoing academic and scientific exchanges that involve both students and researchers working at the cutting edge of modern physics.
Turning the LHC Into a Gluon Microscope
To examine the subtle spatial variations and distributions of gluons hidden deep inside atomic nuclei, the research team utilized an intricate technique known as incoherent J/ψ photonuclear production. This specialized methodology leverages the unique environment created inside CERN’s flagship particle accelerator, the Large Hadron Collider, during its high-energy operational runs.
"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 explained. "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."
Historically, many traditional measurements have been limited by averaging the gluon distribution across an entire atomic nucleus, masking local variations. Incoherent J/ψ production, by contrast, possesses the unique capability to reveal localized changes in gluon density. This analytical power makes it possible for physicists to investigate subatomic structures that are significantly smaller than a proton itself.
The powerful gluon fields residing inside atomic nuclei are central to the structural makeup of nearly all visible matter in the universe, yet physicists have long struggled to fully understand how massive numbers of gluons behave collectively under extreme conditions. The latest experimental approach offers a dramatic technological and analytical leap forward for the field.
"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."
To put these unfathomable scales into a relatable perspective, Tapia Takaki offered a macroscopic analogy. If an atomic nucleus were to be magnified to the colossal size of a professional football stadium, the experiment’s highest resolution would be sharp enough to clearly distinguish distinct features measuring only a few yards wide across the playing field.
"At these extraordinary scales, we observe evidence that the gluons begin to behave collectively, a phenomenon known as gluon saturation," he added, highlighting the profound implications of the data.
Gluon Hot Spots Come Into Focus
For years, Tapia Takaki has helped pioneer this specific experimental approach while contributing actively to theoretical frameworks in which gluons gather into localized zones of exceptionally high density, frequently characterized by physicists as "hot spots." Within the framework of the energy-dependent hot-spot model, these dense regions evolve dynamically as collision energy increases. Their unique behavior could provide researchers with definitive experimental signatures of previously unexplored physics governed by the strong interaction.
For the purposes of this new study, the research team measured incoherent J/ψ production across a wide spectrum of photon-nucleus energies, ranging from 20 billion to 633 billion electron volts. Furthermore, the scientists meticulously studied how the production process varied in response to momentum transfer, a critical variable that dictates the precise spatial scale being probed deep within the atomic nucleus.
"The results revealed a striking pattern," the University of Kansas researcher noted. "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
This unexpected and pronounced suppression presents a formidable challenge to a long-standing theoretical explanation known as "nuclear shadowing," which has successfully accounted for a wide array of earlier, lower-resolution measurements in nuclear physics.
"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," Tapia Takaki explained.
However, the latest high-precision measurements strongly suggest that conventional nuclear shadowing by itself is insufficient to account for the intricate pattern observed by the ALICE collaboration. Instead, the experimental results align closely with an alternative phenomenon known as gluon saturation, which is fundamentally predicted by quantum chromodynamics, the established theory describing the strong interaction and subatomic particles.
"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 concluded. "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."
