Medieval alchemists sought for centuries to transform lead into gold. This quest, called chrysopoeia, has finally found a scientific answer thanks to the work of the ALICE collaboration at CERN. Physicists have succeeded in converting lead nuclei into gold nuclei using the Large Hadron Collider (LHC), the world's most powerful particle accelerator.
An ultra-fast transmutation process
The process relies on near-collisions between lead nuclei traveling at a dizzying speed of 99.999993% of the speed of light. Unlike head-on collisions that create quark-gluon plasma, reproducing the conditions of the early universe, these indirect interactions generate electromagnetic fields of exceptional intensity.
"It's impressive to see that our detectors are capable of handling head-on collisions producing thousands of particles, while remaining sensitive to collisions in which only a few particles are produced," emphasizes Marco Van Leeuwen, spokesperson for the ALICE project.
The transformation mechanism relies on electromagnetic dissociation. The lead nucleus, which contains 82 protons, produces a particularly intense electromagnetic field. When the nuclei pass close to each other without directly colliding, this field generates short-lived photons that interact with the internal structure of the nuclei. These interactions cause the ejection of a small number of neutrons and protons. To obtain gold, which has 79 protons, exactly three protons must be removed from a lead nucleus.
Measurements reveal that the LHC is currently producing gold at a rate of about 89,000 nuclei per second during lead-lead collisions. However, these gold nuclei only exist for a tiny fraction of a second before immediately fragmenting into protons, neutrons, and other particles upon hitting the walls of the beam pipe.
Between 2015 and 2018, the LHC's four major experiments created approximately 86 billion gold nuclei, with a total mass of just 29 picograms. Despite technical improvements that doubled this production during Run 3, the quantity remains trillions of times less than what would be needed to make a single piece of jewelry.
"Although the dream of medieval alchemists has technically become a reality, their hope of making a fortune is once again fading," the research team observes. The value of these experiments lies not in the commercial production of gold, but in understanding the fundamental processes of matter. This work will enable us to test and improve theoretical models of electromagnetic dissociation, which are essential for optimizing the performance of current and future colliders.
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