Lead was turned into gold - but only for a trillionth of a second

The 17-mile Large Hadron Collider (LHC), located beneath the French-Swiss border, regularly slams heavy ions together at near light speed. But on July 30, 2025, researchers reported something many thought belonged to folklore: lead ions momentarily changed into gold before decaying back into more ordinary matter.

The analysis shows that a single run of lead can produce gold nuclei with a cross section comparable to the total hadronic collision rate. This makes the “modern alchemy” far more common in the tunnel than anyone expected.

“Usually in collider experiments, we make the particles crash into each other to produce lots of debris,” said Daniel Tapia Takaki, professor of physics at the University of Kansas and leader of the group on the ALICE experiment.

His team developed the method that spots what happens when the ions merely graze, an interaction so clean that almost nothing else appears in the detectors besides a flash of light and an altered nucleus.

Gold from lead, briefly

Ultraperipheral collisions happen when two atomic nuclei pass close to each other without touching, but their powerful electromagnetic fields still interact.

Instead of smashing apart, each ion showers the other with a burst of high-energy photons described by the Weizsäcker Williams method, allowing photons from one nucleus to probe or even transform its partner. That photon barrage can knock out one, two, or three protons.

If three protons are lost, the lead-208 nucleus briefly becomes a gold-205 nucleus – fulfilling the alchemist’s dream, though only for about 10⁻²³ seconds. That’s just long enough to leave a signal in the forward calorimeters.

Previous ALICE runs hinted that such clean events existed, but the detector was optimized for messy head on smash ups. Tapia Takaki’s team re-tuned readouts, added vetoes, and refined a two-stage fit to isolate neutron from proton peaks.

What near miss collisions reveal

Because photons carry no net charge, photon photon or photon nucleus interactions are free from the spray of hadronic debris that plagues central collisions. The clean environment allows physicists to study nuclear structure and test QED at previously unreachable energy scales.

The Kansas-led analysis clocked a gold production cross section of 6.8 barns, only 12 percent shy of the 7.67 barn total inelastic rate for ordinary lead-lead interactions at the same energy.

That means every time the LHC delivers a hadronic ion collision, there is roughly another event nearby where a lead ion quietly becomes gold and then disintegrates.

The same data set pinned the 0 proton channel at 157.5 barns, the 1 proton channel at 40.4 barns, and the 2 proton channel at 16.8 barns. These results matched or exceeded theoretical predictions from the RELDIS photonuclear model within 25 percent.

Discrepancies suggest existing models poorly capture pre-equilibrium emission and nucleon coalescence in single proton channels.

Tracking alchemy at light speed

The ALICE collaboration relies on zero degree calorimeters placed 369 feet downstream of the interaction point to record neutral and charged fragments.

The KU team gated on events where proton energy fell within two standard deviations of the beam energy and at least one neutron hit the neighboring neutron calorimeter. This isolated a data set of just two million events from 2.05 million triggers.

They corrected for acceptance, efficiency, and the small chance that a peripheral hadronic collision could imitate an electromagnetic event.

Monte Carlo studies using RELDIS and the AAMCC-MST transport code showed that hadronic imposters contribute less than one percent to the single proton sample, leaving the photon driven signal essentially pure.

The resulting fit revealed broad 1 proton and broad 2 proton peaks, about twice as wide as the corresponding neutron peaks. This is because relativistic protons can leak energy at the calorimeter edges or through interactions with beam line material.

A modified Gaussian model with width scaling by proton count corrected smearing – now adopted by other heavy ion groups.

Flash matters for future colliders

Removing three protons turns lead into gold, but knocking out even one proton turns the ion into thallium, which bends differently in the LHC magnets.

Secondary particle beams that aren’t properly controlled can hit cold components, shut down superconducting magnets, or trigger safety systems. These issues could limit the performance of future 27 TeV upgrades and the proposed 100-km Future Circular Collider.

By measuring the full suite of 0- to 3- proton channels, the ALICE team provides essential inputs for loss maps that machine engineers use to design collimators and shielding.

The data also feed into simulations for the U.S. Electron Ion Collider (EIC), where understanding photon induced breakup of nuclei is critical for background rejection in precision measurements.

More than just gold

Beyond gold, near-miss collisions can produce mercury, thallium, or platinum isotopes – each with unique decay paths and insights.

Light-by-light scattering, axion-like particle searches, and studies of nuclear excitation all benefit from knowing exactly how often and how cleanly these channels occur.

Tapia Takaki noted that the study may be crucial for designing the next generation of machines, because every lost beam ion costs days of accelerator time and significant operational money.

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 the analysis to four and five proton emissions once Run 3 data arrive, pushing sensitivity toward nuclei around hafnium and tantalum.

They’re working with theorists to refine photonuclear models so neutron-to-proton ratios better match observations.

A dedicated trigger for ultraperipheral collisions is in development. It combines existing calorimeter logic with real-time machine learning filters to capture rare events without overwhelming the data acquisition system.

If successful, physicists could watch modern alchemy unfold almost as it happens, perhaps even identifying long lived isomers before they decay in flight.

The study is published in the journal Physical Review C.

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