Nuclear physicists working at the Large Hadron Collider recently made headlines by achieving the centuries-old dream of alchemists (and nightmare of precious-metals investors): They transformed lead into gold.
At least for a fraction of a second. The scientists reported their results in Physical Reviews.
The accomplishment at the Large Hadron Collider, the 17-mile particle accelerator buried under the French-Swiss border, happened within a sophisticated and sensitive detector called ALICE, a scientific instrument roughly the size of a McMansion.
It was scientists from the University of Kansas, working on the ALICE experiment, who developed the technique that tracked “ultra-peripheral” collisions between protons and ions that made gold in the LHC.
“Usually in collider experiments, we make the particles crash into each other to produce lots of debris,” said Daniel Tapia Takaki, professor of physics and leader of KU’s group at ALICE. “But in ultra-peripheral collisions, we’re interested in what happens when the particles don’t hit each other. These are near misses. The ions pass close enough to interact — but without touching. There’s no physical overlap.”
The ions racing around the LHC tunnel are heavy nuclei with many protons, each generating powerful electric fields. When accelerated, these charged ions emit photons — they shine light.
“When you accelerate an electric charge to near light speeds, it starts shining,” Tapia Takaki said. “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 said during these UPC “flashes” surprising interactions can occur, including the rate event that sparked worldwide attention.
“Sometimes, the photons from both ions interact with each other — what we call photon-photon collisions,” he said. “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 designed to collect data on head-on collisions that result in messy sprays of particles.
“These clean interactions were hard to detect with earlier setups,” Tapia Takaki said. “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 methods allowed for the news-making discovery that the LHC team transmuted lead into gold momentarily via ultra-peripheral collisions where lead ions lose three protons (turning the speck of lead into a gold speck) for a fraction of a second.
Tapia Takaki’s KU co-authors on the paper are graduate student Anna Binoy; graduate student Amrit Gautam; postdoctoral researcher Tommaso Isidori; postdoctoral research assistant Anisa Khatun; and research scientist Nicola Minafra.
The KU team at the LHC ALICE experiment plans to continue studying the ultra-peripheral collisions. Tapia Takaki said that while the creation of gold fascinated the public, the potential of understanding the interactions goes deeper.
“This light is so energetic, it can knock protons out of the nucleus,” he said. “Sometimes one, sometimes two, three or even four protons. We can see these ejected protons directly with our detectors.”
Each proton removed changes the elements: One gives thallium, two gives mercury, three gives gold.
“These new nuclei are very short-lived,” he said. “They decay quickly, but not always immediately. Sometimes they travel along the beamline and hit parts of the collider — triggering safety systems.”
That’s why this research matters beyond the headlines.
“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,” Tapia Takaki said. “This ‘alchemy’ may be crucial for designing the next generation of machines.”
This work was supported by the U.S. Department of Energy Office of Science, Office of Nuclear Physics.