For scientists studying matter at extreme energies, understanding how large atomic cores like lead break apart when they pass near each other—without actually colliding—is a vital way to learn about the forces that govern the universe. These rare interactions, where lead atoms influence each other through their powerful electric fields alone, offer a unique opportunity to observe how energy from light particles called photons can disrupt atomic structures. Researchers decided to examine more closely how protons, which are positively charged particles found in the nucleus, are released during this process, with the goal of improving the models that describe such interactions and aiding the development of future research facilities like the Electron-Ion Collider, a next-generation machine for studying atomic nuclei.
Under ALICE Collaboration umbrella, the research team working with the Large Ion Collider Experiment, a major particle physics initiative at the European Organization for Nuclear Research, gathered data using the experiment’s advanced detection systems at the world’s largest particle accelerator, known as the Large Hadron Collider. They conducted the first detailed investigation of events in which protons were emitted alongside neutrons, which are neutral particles also located in the atomic nucleus, when lead atoms passed near one another at high speed. Their findings, published in Physical Review C, described how different combinations of particles were released and compared these observations to forecasts made by a widely used simulation tool known as the Relativistic Electromagnetic Dissociation model, which estimates how atomic nuclei break apart under the influence of electric forces.
Most of the time, these break-apart events did not result in the emission of protons, confirming that such outcomes are relatively rare. However, when protons were emitted, the patterns were clearly observable. The team found that the model closely matched observed events where no protons or multiple protons were emitted together. Nevertheless, it appeared to underestimate the frequency of events involving one or two protons. The researchers also analyzed cases where a single proton was emitted together with one, two, or three neutrons and discovered that the model tended to overestimate the frequency of such events.
Perhaps most notably, the way these particles were emitted appeared to align with the creation of new forms of chemical elements. When only neutrons were released, different versions of lead, called isotopes, were formed. When one or more protons were emitted, the resulting elements included thallium, mercury, and gold. These findings help scientists better understand how components of atoms are rearranged during these interactions and what types of new matter might emerge. As Dr. Acharya explained, “The Relativistic Electromagnetic Dissociation model suggests these proton and neutron emissions are linked with the production of elements like thallium and gold, which we now observe with greater clarity.”
With the help of highly sensitive detectors positioned to capture particles moving at steep angles, the team measured both protons and neutrons with high accuracy. The detectors specifically designed to measure protons were aligned directly with the path of the lead atom beam, while others were used to detect neutrons. The scientists employed a careful statistical method—using patterns and probabilities in the collected data—to interpret the energy readings from these devices. This approach enabled them to identify the events relevant to their study. They also made necessary adjustments to their analysis to account for particles that may have gone undetected or been misidentified. Because protons tend to lose more energy and travel differently than neutrons, this part of the research was particularly important.
These findings enhance our understanding of how large atomic structures break apart when influenced by the electric fields of nearby atoms. Simultaneously, the study challenges parts of the Relativistic Electromagnetic Dissociation model, showing that although it remains a valuable tool, improvements are needed. As Dr. Acharya noted, “These results serve as a benchmark for theoretical models and support the design of future facilities where understanding such dissociation processes is vital.”
Looking at the bigger picture, this research connects experimental data with the predictions made by computer simulations, which are digital models used to replicate physical phenomena. The work of the Large Ion Collider Experiment group marks meaningful progress in nuclear science. It provides a clearer understanding of how lead atoms behave under extreme conditions and sheds light on how atomic components are reassembled into new materials in both outer space and laboratory environments.
Journal Reference
S. Acharya et al., “Proton emission in ultraperipheral Pb-Pb collisions at √sNN = 5.02 TeV,” Physical Review C, 2025. DOI: https://doi.org/10.1103/PhysRevC.111.054906
About the Authors
The ALICE (A Large Ion Collider Experiment) collaboration is a major international research group based at the European Organization for Nuclear Research (CERN). It focuses on studying the behavior of matter under extreme conditions, particularly the properties of quark-gluon plasma—a state of matter thought to have existed just after the Big Bang. Using the powerful particle collisions generated by the Large Hadron Collider, ALICE investigates how atomic nuclei break apart and reform when exposed to incredibly high temperatures and energy densities. The collaboration includes hundreds of scientists and engineers from institutions around the world, all working together to explore the fundamental building blocks of the universe. ALICE’s advanced detection systems are specially designed to analyze heavy-ion collisions, such as those involving lead nuclei, providing insights into the strong force that binds protons and neutrons together. The project plays a vital role in advancing our understanding of nuclear physics and the early universe.
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