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The ALICE collaboration at CERN’s Large Hadron Collider (LHC) has experimentally observed the conversion of lead nuclei into gold nuclei through a rare nuclear transmutation process. This discovery, reported in Physical Review C, marks the first systematic detection of gold production via electromagnetic dissociation at the LHC.

Historical Context: The Dream of Chrysopoeia

The idea of turning lead, a common base metal, into gold—a precious metal—dates back to medieval alchemy and is known as chrysopoeia. Alchemists were motivated by the similarity in density between lead and gold, but modern chemistry has established that chemical reactions cannot change one element into another since elements differ by their number of protons.

With the advent of nuclear physics in the 20th century, scientists learned that nuclear reactions could transmute elements, either naturally (radioactive decay) or artificially (particle accelerators). However, the ALICE experiment at CERN has revealed a novel mechanism of such transmutation involving near-miss collisions of lead nuclei.

Mechanism of Transmutation at the LHC

The LHC, located on the Franco-Swiss border near Geneva, is the world’s largest and most powerful particle accelerator. It accelerates two beams of lead nuclei (each containing 82 protons) to velocities close to the speed of light (about 99.999993% of the speed of light) inside a 27-kilometre ring.

During ultra-peripheral or near-miss collisions, lead nuclei pass close by each other without direct contact. The intense electromagnetic fields generated by the 82 protons in each lead nucleus compress into a short-lived pulse of photons, producing a process called electromagnetic dissociation.

This photon-induced excitation causes the nucleus to oscillate internally and emit a small number of protons and neutrons. When a lead nucleus (Pb-208) ejects three protons and two neutrons, it effectively becomes a gold nucleus (Au-203).

Role of ALICE Detector and Zero Degree Calorimeters (ZDCs)

The ALICE detector uses Zero Degree Calorimeters (ZDCs) to detect and count these nuclear dissociation events by measuring the number of emitted protons and neutrons. The emission of zero, one, two, or three protons corresponds to the creation of lead, thallium, mercury, and gold nuclei, respectively.

This capability allows ALICE to distinguish rare electromagnetic transmutations from the usual high-energy head-on collisions producing thousands of particles.

Key Findings and Quantitative Data

• The LHC produces gold nuclei at a rate of about 89,000 nuclei per second during lead–lead collisions.
• During LHC Run 2 (2015–2018), approximately 86 billion gold nuclei were generated.
• This corresponds to a mass of only 29 picograms (2.9 × 10?¹¹ grams), far too little for any practical use like jewelry.
• Run 3, with increased luminosity, has nearly doubled the amount of gold produced.
• The gold nuclei exist only for a fraction of a second before fragmenting into protons, neutrons, and other particles upon hitting the LHC’s beam pipe or collimators.

Significance

This discovery:

• Demonstrates a new pathway for nuclear transmutation via electromagnetic interactions at ultra-relativistic speeds.
• Provides experimental data that helps refine theoretical models of electromagnetic dissociation.
• Aids in understanding and predicting beam losses, which are critical for improving the performance of the LHC and future particle colliders.

About CERN and the LHC

• CERN (European Organisation for Nuclear Research), established in 1954, is Europe’s premier high-energy physics research organization with 23 member states and 10 associate members (including India).
• The LHC accelerates particles to nearly light speed in a 27-km circular tunnel under the Franco-Swiss border, facilitating collisions studied by four major experiments: ALICE, ATLAS, CMS, and LHCb.
• These experiments aim to probe fundamental particles and forces, test the Standard Model of particle physics, and explore conditions of the early universe.