Gluons are the particles responsible for carrying the strong nuclear force, which binds quarks together inside protons and neutrons. Although quarks are often presented as the basic constituents of matter, gluons account for nearly all of the mass of ordinary visible objects through the energy stored in the strong interaction. Their behavior becomes especially complex at high energies, where the number of gluons inside a nucleus can increase dramatically. Understanding how these particles are arranged, fluctuate and interact is one of the central challenges of quantum chromodynamics, the theory describing the strong force.

The ALICE collaboration investigated this problem using data collected during Run 2 of the Large Hadron Collider. Instead of colliding lead nuclei directly, researchers studied ultra-peripheral encounters, in which the nuclei passed extremely close to one another while remaining physically separated. The intense electromagnetic fields surrounding the rapidly moving lead ions acted as sources of high-energy photons. When a photon from one nucleus interacted with the other, it could produce a short-lived J/ψ particle, a bound state made of a charm quark and its antimatter partner.

J/ψ production provides a highly sensitive probe of gluons because the process depends directly on the gluon field inside the target nucleus. The “incoherent” form of the reaction is particularly valuable: rather than responding only to the average shape of the entire nucleus, it is sensitive to local fluctuations in gluon density. In effect, the process can reveal smaller structures inside the nucleus, including concentrated regions sometimes described as gluon “hot spots.”

The experiment measured J/ψ production over photon–nucleus energies ranging from 20 to 633 billion electron volts. At the same time, it tracked momentum transfer, a quantity that determines the spatial resolution of the measurement. Larger momentum transfer allows researchers to examine finer features, much as a microscope resolves smaller structures by using a shorter wavelength. ALICE studied the gluon field at approximate resolutions of 0.6, 0.3 and 0.2 femtometers, with the finest scale corresponding to structures roughly one-quarter the diameter of a proton.

The result revealed a striking suppression of J/ψ production at the smallest spatial scales explored. The effect reached a statistical significance of about three standard deviations, meaning the probability that it arose from an ordinary fluctuation is low enough to attract serious theoretical attention. The observation is difficult to reconcile with models based solely on nuclear shadowing, the traditional explanation for why gluon-related processes can be weakened inside a nucleus.

Nuclear shadowing describes a situation in which gluons from different nucleons overlap and interfere, reducing the effective probability of certain interactions. The phenomenon resembles layers of partially transparent material blocking one another: the gluon field remains present, but its contribution to a measurable process is diminished. Shadowing has successfully described many earlier observations, but the new ALICE measurement indicates that it may not capture the full behavior of gluons when the experiment probes exceptionally small distances.

A competing explanation is gluon saturation. In this regime, gluons become so numerous that their mutual interactions prevent their density from increasing indefinitely. Instead of continuing to multiply freely, the gluon field reaches a high-density state in which nonlinear effects become essential. Gluons can merge, interact and redistribute their energy, producing a collective system rather than a simple sum of independent particles. The suppression observed by ALICE is consistent with this picture and could represent an important experimental sign of saturation inside ordinary atomic nuclei.

Tapia Takaki has played a pioneering role in developing the hot-spot approach used to describe these phenomena. In such models, gluons are not spread smoothly throughout a nucleus but are concentrated in localized, fluctuating regions whose size and energy dependence can change during an interaction. By combining energy-dependent measurements with momentum-transfer information, researchers can test whether these regions evolve as predicted and determine whether collective gluon dynamics are becoming visible. The new data provide a stronger way to separate competing models than measurements based on a single energy or spatial scale.

The findings do not yet establish every detail of gluon saturation, and additional measurements will be needed to confirm the interpretation. However, they mark a significant advance in the effort to map the gluonic structure of matter. By turning the LHC into a high-resolution microscope for nuclear fields, ALICE is approaching a regime where the strong force may reveal its most collective behavior. If future analyses strengthen the present result, physicists could gain direct evidence that the dense gluon fields inside nuclei undergo a transition from individual-particle dynamics to a coordinated quantum state—one that helps explain how the visible universe acquires its mass and structure.

Subject of Research: Gluon behavior and saturation inside atomic nuclei, studied through incoherent J/ψ photonuclear production.

Web References: Physical Review Letters study; CERN Large Hadron Collider

References: DOI: 10.1103/jmwb-75m7

Image Credits: CERN

Keywords

ALICE experiment, CERN, Large Hadron Collider, gluons, gluon saturation, nuclear shadowing, J/ψ production, quantum chromodynamics, atomic nuclei, particle physics, high-energy physics

The LHC, an immense 17-mile-long particle accelerator straddling the French-Swiss border, is famous for smashing subatomic particles together at near-light speeds to better understand the universe’s fundamental components. Traditionally, its experiments have focused on head-on collisions, resulting in cascades of diverse particles. However, the recent discovery centers on ultra-peripheral collisions (UPCs), a subtler interaction where particles pass extremely close to each other without direct contact. This method leverages the powerful electromagnetic fields surrounding heavy atomic nuclei like lead.

Ultra-peripheral collisions occur when ions—highly charged nuclei racing around the collider’s tunnel—glance past each other without overlapping. Each ion’s tremendous electric field produces a stream of photons, particles of light that can interact with neighboring ions in unique ways. When these photons collide or interact with a nucleus, they can induce transformations that are impossible in more typical collider environments. The ALICE detector is specially designed to capture these nuanced interactions, albeit requiring novel analytical techniques due to the inherently clean and low-multiplicity nature of the events.

At the heart of this research is a team of physicists from the University of Kansas who pioneered methodologies to detect and analyze these ultra-peripheral interactions. Led by Professor Daniel Tapia Takaki, they developed strategies for isolating these “near-miss” collisions, enabling them to peer into phenomena where ions exchange or lose protons without the violent debris typical of conventional collisions. This clean experimental setup allowed them to witness lead nuclei shedding three protons, temporarily becoming gold nuclei.

“Typically, particle collisions create showers of hundreds or thousands of new particles, making it difficult to isolate rare processes,” explains Tapia Takaki. “In contrast, ultra-peripheral collisions are like cosmic whispers—rare and pristine interactions revealing delicate aspects of nuclear structure and electromagnetic behavior seldom seen before.” This innovative approach required refining the ALICE detector’s data acquisition and analysis protocols to filter out background noise and reliably identify these fleeting transmutations.

The physics behind this transmutation is embedded in the interplay of photons and nuclear forces. When two lead ions accelerate to relativistic speeds, each ion emits a flux of virtual photons due to its electric charge. In ultra-peripheral collisions, these photons can collide head-on, triggering photon-photon interactions. Such interactions can produce pairs of new particles or cause nuclei to eject protons, altering their elemental identity. The specific loss of three protons transforms a lead nucleus (with atomic number 82) into gold (atomic number 79), albeit for an extraordinarily brief period before it decays.

Beyond the poetic allure of turning lead into gold, the implications for particle physics and future collider design are profound. As plans emerge for next-generation accelerators—some spanning up to 100 kilometers—understanding the spectrum of nuclear byproducts produced in ultra-peripheral collisions becomes essential. These transient nuclei, although short-lived, can interact with collider components, triggering safety mechanisms and potentially affecting machine performance. The insights gained from ALICE’s UPC studies will inform both theoretical models and practical engineering considerations.

Moreover, ultra-peripheral collisions offer a comparatively “clean” way to probe nuclear structure and electromagnetic interactions with less background clutter than conventional collision events. This facilitates precision measurements of nuclear excitations, photon-induced reactions, and the behavior of quarks and gluons inside nuclei under electromagnetic influence. Such precision is crucial for refining the Standard Model of particle physics and exploring potential new physics phenomena beyond current theoretical frameworks.

The ALICE collaboration’s discovery wouldn’t have been possible without a team of dedicated researchers who meticulously developed the analytical tools to tease out these rare events from vast datasets. Graduate students Anna Binoy and Amrit Gautam, postdoctoral researchers Tommaso Isidori and Anisa Khatun, and research scientist Nicola Minafra significantly contributed to the analysis and interpretation of the data, strengthening KU’s vital role in the international collaboration.

This research, formally published in Physical Review C, is a testament to decades of cumulative expertise and innovation within the nuclear physics community. The support from the U.S. Department of Energy’s Office of Science, particularly the Office of Nuclear Physics, underscores the strategic importance of advancing fundamental science that bridges multiple fields: from high-energy physics to nuclear engineering.

While the transient alchemy captured global headlines, the broader scientific narrative is one of uncovering nuanced interactions that push the boundaries of what particle accelerators can reveal. Much like a cosmic flashbulb illuminating the heart of atomic nuclei, these photon-induced UPCs are reshaping our toolkit for interrogating the subatomic world. The continued exploration of UPCs promises to yield insights into matter under extreme electromagnetic fields, nuclear deformation, and possibly even phenomena linked to the early universe’s conditions.

Looking forward, the team anticipates that refining UPC measurement techniques and upgrading detector capabilities will deepen our understanding of how light interacts with matter at the smallest scales. Such breakthroughs have cascading effects, from guiding the construction of future collider facilities to informing astrophysical models where high-energy photon interactions play a starring role, such as in neutron stars and cosmic ray propagation.

In sum, the University of Kansas-led investigation at CERN’s ALICE detector reveals a stunning natural phenomenon: the ephemeral transformation of lead into gold. Beyond the fantastical headline, this experiment enriches the foundations of nuclear physics and opens new avenues for research into photon-mediated nuclear processes. As particle physics ventures into ever higher energies and complexities, this elegant exploitation of ultra-peripheral collisions symbolizes the subtle yet profound discoveries still awaiting in the quantum frontier.

Subject of Research: Ultra-peripheral collisions and transient nuclear transmutation at the Large Hadron Collider.

Article Title: [Not provided in the original content.]

News Publication Date: [Not explicitly stated.]

Web References:

• ALICE experiment overview: https://home.cern/science/experiments/alice
• Physical Review C article DOI: 10.1103/PhysRevC.111.054906

References:

• Tapia Takaki et al., “Photon-induced reactions in ultra-peripheral lead collisions at the LHC,” Phys. Rev. C 111, 054906 (2023).

Image Credits: Credit: CERN

Keywords

Large Hadron Collider, ALICE experiment, ultra-peripheral collisions, nuclear physics, photon-photon collisions, nuclear transmutation, lead to gold, particle accelerator, proton ejection, nuclear byproducts, high-energy photons, nuclear structure