In a groundbreaking development that echoes the age-old aspirations of alchemists, nuclear physicists working at CERN’s Large Hadron Collider (LHC) have momentarily transformed lead into gold. This extraordinary feat, achieved through ultra-peripheral collisions within the LHC’s ALICE experiment, represents a remarkable fusion of theoretical physics and experimental prowess. The transformation, albeit fleeting and confined to fractions of a second, opens new vistas in our understanding of nuclear interactions and particle 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.]
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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
