Unbelievable, but true: scientists have achieved the seemingly impossible, turning lead into gold! This groundbreaking discovery was made at the Large Hadron Collider (LHC), an impressive 17-mile-long facility located beneath the French-Swiss border. On July 30, 2025, researchers witnessed a remarkable phenomenon: lead ions briefly transformed into gold before returning to their original state.
The analysis revealed an astonishing fact: a single run of lead ions can produce gold nuclei with a cross-section comparable to the total hadronic collision rate. This "modern alchemy" is more common than anyone anticipated.
Daniel Tapia Takaki, a physics professor at the University of Kansas, led the team behind this groundbreaking experiment. He explained that in typical collider experiments, particles are made to collide, creating a lot of debris. However, his team developed a method to observe what happens when ions merely graze each other, resulting in a clean interaction with minimal debris and an altered nucleus.
"Gold from lead, briefly" is a fascinating concept. Ultraperipheral collisions occur when atomic nuclei pass close to each other without touching, but their powerful electromagnetic fields interact. Instead of smashing apart, the ions exchange a burst of high-energy photons, allowing photons from one nucleus to probe or even transform the other. This photon barrage can remove one, two, or three protons, and when three protons are lost, the lead-208 nucleus becomes a gold-205 nucleus for a fleeting moment of about 10^-23 seconds.
The ALICE experiment, led by Tapia Takaki's team, optimized their detectors to capture these clean events. They reconfigured readouts, added vetoes, and refined their methods to isolate neutron and proton peaks.
"What near miss collisions reveal" is a crucial aspect of this research. Photons, being neutral, produce clean interactions without the hadronic debris seen in central collisions. This clean environment allows physicists to study nuclear structure and test QED at previously unreachable energy scales.
The Kansas-led analysis found a gold production cross-section of 6.8 barns, which is remarkably close to the total inelastic rate for ordinary lead-lead interactions at the same energy. This means that for every hadronic ion collision, there's another nearby event where a lead ion becomes gold and then disintegrates.
The data also revealed insights into the 0, 1, and 2 proton channels, with results matching or exceeding theoretical predictions. However, discrepancies suggest that existing models need improvement, especially regarding pre-equilibrium emission and nucleon coalescence in single proton channels.
"Tracking alchemy at light speed" is an exciting prospect. The ALICE collaboration uses zero-degree calorimeters to record neutral and charged fragments. By gating on events with specific proton energy levels and neutron detections, the KU team isolated a valuable dataset. They corrected for various factors and conducted Monte Carlo studies to ensure the purity of their photon-driven signal.
The resulting fit revealed broad proton peaks, indicating the challenges of detecting relativistic protons. A modified Gaussian model, now adopted by other heavy ion groups, corrected for this smearing.
"Flash matters for future colliders" highlights the importance of controlling secondary particle beams. Removing even one proton from an ion can change its behavior in the LHC magnets, potentially causing issues with cold components, superconducting magnets, and safety systems. This knowledge is crucial for the performance of future upgrades and proposed colliders like the 100-km Future Circular Collider.
By measuring the full suite of proton channels, the ALICE team provides essential inputs for machine engineers to design collimators and shielding. This data is also vital for simulations of the U.S. Electron Ion Collider, where understanding photon-induced breakup of nuclei is critical for precision measurements.
"More than just gold" emphasizes the broader implications of near-miss collisions. These events can produce mercury, thallium, and platinum isotopes, each offering unique decay paths and insights. Light-by-light scattering, axion-like particle searches, and nuclear excitation studies all benefit from precise knowledge of these channels.
Tapia Takaki emphasized the significance of this study for designing future colliders. Every lost beam ion represents a significant cost in terms of accelerator time and operational expenses. Catching a glimpse of gold is not just about wealth; it's about ensuring the safe and efficient operation of billion-dollar facilities.
"Next steps for gold physics" outline the team's plans. They aim to extend their analysis to four and five proton emissions when Run 3 data becomes available, exploring nuclei around hafnium and tantalum. They're collaborating with theorists to refine photonuclear models and improve neutron-to-proton ratio predictions.
A dedicated trigger for ultraperipheral collisions is in development, combining calorimeter logic with machine learning filters to capture rare events efficiently. If successful, physicists may witness modern alchemy in real-time, potentially identifying long-lived isomers before they decay.
This groundbreaking study was published in Physical Review C, offering a glimpse into the future of particle physics and the potential for further discoveries.
