Nuclear scientists at the Large Hadron Collider recently made headlines by achieving what alchemists dreamed of for centuries (and what would give precious metals investors nightmares): turning lead into gold, at least for a split second. The findings were published in Physical Reviews.
ALICE experiment at CERN's Large Hadron Collider, where KU nuclear physicists helped detect gold, briefly, during ultra-peripheral collisions. Image Credit: CERN
The achievement at the Large Hadron Collider, the 17-mile particle accelerator buried beneath the French-Swiss border, occurred within a sophisticated and sensitive detector called ALICE.
Scientists at the University of Kansas, who were working on the ALICE experiment, developed the technology for tracking “ultra-peripheral” collisions between protons and ions that produced gold in the LHC.
Usually in collider experiments, we make the particles crash into each other to produce lots of debris. But in ultra-peripheral collisions, we are interested in what happens when the particles do not hit each other. These are near misses. The ions pass close enough to interact but without touching. There is no physical overlap.
Daniel Tapia Takaki, Professor, University of Kansas
The ions racing around the LHC tunnel are heavy nuclei with numerous protons, each of which generates a tremendous electric field. When accelerated, these charged ions generate photons, which produce light.
Takaki added, “When you accelerate an electric charge to near light speeds, it starts shining. One ion can shine light that essentially takes a picture of the other. When that light is energetic enough, it can probe deep inside the other nucleus, like a high-energy flashbulb.”
The KU researcher stated that during these UPC “flashes,” unexpected interactions might occur, such as the rate incident that drew global attention.
Takaki stated, “Sometimes, the photons from both ions interact with each other what we call photon-photon collisions. These events are incredibly clean, with almost nothing else produced. They contrast with typical collisions where we see sprays of particles flying everywhere.”
However, the ALICE detector and the LHC were built to capture data on head-on collisions, which produce messy particle sprays.
Takaki remarked, “These clean interactions were hard to detect with earlier setups. Our group at KU pioneered new techniques to study them. We built up this expertise years ago when it was not a popular subject.”
These technologies enabled the LHC team to make the groundbreaking finding that lead ions lost three protons in ultra-peripheral collisions, transforming the lead speck into a gold speck for a fraction of a second.
Research scientist Nicola Minafra, postdoctoral researcher Tommaso Isidori, postdoctoral research assistant Anisa Khatun, graduate student Anna Binoy, and graduate student Amrit Gautam are Tapia Takaki’s KU co-authors on the study.
The ultra-peripheral collisions will be further investigated by the KU team at the LHC ALICE experiment. According to Tapia Takaki, while the public was fascinated by the idea of producing gold, the real significance lies in gaining a deeper understanding of the underlying particle interactions.
Tapia Takaki explained, “This light is so energetic, it can knock protons out of the nucleus. Sometimes one, sometimes two, three, or even four protons. We can see these ejected protons directly with our detectors.”
The elements vary with each proton removed: thallium is produced by one, mercury by two, and gold by three.
Takaki added, “These new nuclei are very short-lived. They decay quickly, but not always immediately. Sometimes they travel along the beamline and hit parts of the collider, triggering safety systems.”
This is why this research holds importance beyond the headlines.
Tapia Takaki stated, “With proposals for future colliders even larger than the LHC, some up to 100 kilometers in Europe and China, you need to understand these nuclear byproducts. This ‘alchemy’ may be crucial for designing the next generation of machines.”
The Office of Nuclear Physics, Office of Science, US Department of Energy, provided support for this study.
Journal Reference:
Acharya, S., et al. (2025) Proton emission in ultraperipheral Pb-Pb collisions at √sNN=5.02 TeV. Physical Review C. doi.org/10.1103/physrevc.111.054906.