The Large Hadron Collider (LHC), a marvel of engineering straddling the French-Swiss border, has achieved a feat once relegated to the realm of alchemy: the temporary transmutation of lead into gold. Researchers reported this unexpected outcome on , demonstrating that, under specific conditions, lead ions can briefly transform before decaying back into more common matter.
The process, observed within the ALICE experiment at the LHC, isn’t the direct, sustained conversion dreamed of by historical alchemists. Instead, it occurs during “ultra-peripheral collisions” – near misses between lead ions where the nuclei don’t directly impact each other, but their powerful electromagnetic fields interact. Analysis indicates that the rate of this lead-to-gold conversion is surprisingly common within the LHC’s operations, with a cross section comparable to the total rate of standard hadronic collisions.
Gold from Lead, Briefly
Traditionally, collider experiments focus on head-on collisions designed to create a shower of debris for analysis. However, the team led by Daniel Tapia Takaki, professor of physics at the University of Kansas, developed a technique to identify and track these ultra-peripheral collisions. These interactions are remarkably clean, producing minimal extraneous particles and allowing for precise observation of the altered nuclei. “Usually in collider experiments, we make the particles crash into each other to produce lots of debris,” explained Tapia Takaki. “But in ultra-peripheral collisions, we’re interested in what happens when the particles don’t hit each other.”
The mechanism relies on the exchange of high-energy photons between the lead nuclei. These photons, described by the Weizsäcker Williams method, can knock out protons from the lead nucleus. Losing three protons transforms a lead-208 nucleus into a gold-205 nucleus, albeit for an incredibly short duration – approximately 10-23 seconds. This fleeting existence is just long enough to register a signal in the LHC’s detectors.
Previous ALICE runs provided hints of these events, but the detector wasn’t optimized for their detection. Tapia Takaki’s team re-tuned the detector’s readouts, implemented vetoes to filter out unwanted signals, and refined a two-stage fitting process to distinguish between neutron and proton peaks, ultimately enabling the clear identification of the gold nuclei.
What Near-Miss Collisions Reveal
The significance of these ultra-peripheral collisions extends beyond simply creating gold. Because photons carry no electric charge, photon-photon or photon-nucleus interactions are free from the “spray of hadronic debris” that complicates analysis in standard collisions. This clean environment allows physicists to study nuclear structure and test quantum electrodynamics (QED) at energy scales previously unattainable.
The Kansas-led analysis measured a gold production cross section of 6.8 barns, only 12 percent lower than the 7.67 barn total inelastic rate for ordinary lead-lead interactions at the same energy. This suggests that for every standard collision occurring within the LHC, a similar event is likely happening nearby where a lead ion is briefly converted into gold before disintegrating.
Further analysis revealed cross sections for the loss of one proton (157.5 barns) and two protons (40.4 and 16.8 barns respectively). These results largely aligned with theoretical predictions from the RELDIS photonuclear model, with discrepancies of less than 25 percent. These minor deviations suggest that current models may not fully capture the complexities of pre-equilibrium emission and nucleon coalescence during these interactions.
Tracking Alchemy at Light Speed
The ALICE collaboration utilizes zero-degree calorimeters, positioned 369 feet downstream from the interaction point, to record the fragments produced during these collisions. The KU team focused on events where the energy of the emitted protons closely matched the beam energy and at least one neutron was detected by a neighboring calorimeter. This selection process yielded a dataset of approximately two million events from 2.05 million triggers.
The team meticulously corrected for factors such as detector acceptance, efficiency, and the possibility of misidentification, where a peripheral hadronic collision might mimic an electromagnetic event. Monte Carlo studies, using RELDIS and the AAMCC-MST transport code, confirmed that such imposters accounted for less than one percent of the single proton sample, ensuring the signal’s purity.
The resulting analysis revealed broad peaks corresponding to the loss of one and two protons, slightly wider than the peaks observed for neutron emissions. This difference is attributed to relativistic effects, where protons can lose energy at the calorimeter edges or through interactions with the beam line materials. A modified Gaussian model, scaling the width based on the number of protons lost, was implemented to correct for this smearing effect and has since been adopted by other heavy ion research groups.
Flash Matters for Future Colliders
While converting lead into gold is a remarkable demonstration of physics, the implications extend to the practical operation of particle colliders. Losing protons from lead ions can transform them into thallium, which behaves differently within the LHC’s magnetic fields. Uncontrolled secondary particle beams can potentially damage sensitive components, trigger safety systems, or even quench superconducting magnets.
These concerns are particularly relevant for future upgrades to the LHC, including a planned increase to 27 TeV, and the proposed 100-km Future Circular Collider. By accurately measuring the rates of 0- to 3-proton emission channels, the ALICE team provides crucial data for designing collimators and shielding to mitigate these risks.
The data also informs simulations for the U.S. Electron Ion Collider (EIC), where understanding photon-induced breakup of nuclei is critical for reducing background noise in precision measurements.
More Than Just Gold
The potential of ultra-peripheral collisions isn’t limited to gold production. These interactions can also generate isotopes of mercury, thallium, or platinum, each offering unique decay paths and insights into nuclear physics. Phenomena like light-by-light scattering, searches for axion-like particles, and studies of nuclear excitation all benefit from a precise understanding of these collision channels.
Tapia Takaki emphasized that the study’s importance lies in ensuring the safe and efficient operation of billion-dollar facilities. “In short, catching a blink of gold is less about getting rich and more about keeping billion dollar facilities running safely and efficiently.”
Next Steps for Gold Physics
The team plans to extend their analysis to include events with four and five proton emissions as data from the LHC’s Run 3 becomes available, pushing the boundaries of sensitivity towards heavier nuclei like hafnium and tantalum. They are also collaborating with theorists to refine photonuclear models, aiming to improve the accuracy of neutron-to-proton ratio predictions. A dedicated trigger for ultra-peripheral collisions is under development, combining existing calorimeter logic with real-time machine learning filters to capture rare events without overwhelming the data acquisition system. If successful, physicists could observe modern alchemy unfolding in near real-time, potentially even identifying long-lived isomers before they decay.
The study was published in the journal Physical Review C.
