Yoctosecond imaging of the 129Xe ground state at the Large Hadron Collider

To understand the nucleus of an atom, scientists have long relied on two very different ways of looking at the world. One approach uses low-energy experiments, where beams of particles gently probe the surface of a nucleus, revealing its average shape and size. The other uses high-energy collisions, smashing atoms together at nearly the speed of light to create a brief, searing fireball of subatomic particles. For decades, these two worlds remained largely separate. The high-energy collisions were seen primarily as a way to study the fundamental forces that hold matter together, while the low-energy methods were the standard for mapping the structure of individual atoms. However, a new perspective has emerged: the violent collision itself can act as a camera, taking a snapshot of the nucleus's internal arrangement before it is destroyed. This is possible because the collision happens so quickly that the positions of the protons and neutrons inside the nucleus are effectively frozen in place during the impact. By analyzing the debris that flies out after the crash, physicists can work backward to reconstruct the arrangement of those particles, effectively imaging the nucleus in a way that was previously impossible.

A team of researchers at CERN has now taken this concept a significant step further, using data from the Large Hadron Collider to create a detailed image of the ground state of the xenon-129 nucleus. In a study published recently, they combined a model of the nucleus as a spinning, deformed object with complex computer simulations of the resulting collision. The goal was to move beyond simply guessing the shape of the nucleus and instead to extract precise, quantitative measurements of how its protons and neutrons are correlated with one another. By analyzing the patterns of particles produced in collisions between xenon atoms, and comparing them to collisions between lead atoms, the team was able to determine that the xenon-129 nucleus is not a perfect sphere, nor a simple oval. Instead, it possesses a complex, three-sided asymmetry, a shape that aligns with theoretical predictions for this specific isotope. This achievement marks a shift from qualitative observation to quantitative measurement, allowing scientists to use particle colliders as precision instruments for mapping the internal geometry of atomic nuclei.

The process relies on the unique conditions created when two heavy ions collide at ultra-relativistic speeds. When a xenon nucleus smashes into another, the interaction happens in less than a yoctosecond, a timescale so short that the protons and neutrons inside cannot move or rearrange themselves during the impact. They are frozen in the configuration they held just before the crash. This frozen configuration is then projected onto the plane of the collision, creating a specific geometric pattern. This pattern dictates how the resulting fireball of hot matter, known as the quark-gluon plasma, expands and cools. If the nucleus is round, the fireball expands evenly. If the nucleus is stretched or flattened, the fireball expands more in some directions than others. By measuring the final flow of particles emerging from this fireball, specifically how they move in relation to each other, physicists can infer the shape of the original nucleus.

The researchers focused on the xenon-129 isotope, which is known to be deformed, meaning it is not a perfect sphere. They used a statistical method called Bayesian inference to analyze a massive dataset of collisions. This method allowed them to test millions of possible shapes and orientations against the experimental data to see which ones matched the observed particle flows. They found that the data strongly supports a model where the xenon nucleus is triaxial, meaning it has three unequal axes, resembling a slightly squashed, irregular shape rather than a simple rugby ball or a pancake. The study determined that the deformation is nearly maximal, with a specific asymmetry angle of approximately 40 degrees. This result is significant because it matches independent calculations made by other theoretical methods, confirming that the high-energy collision approach can reliably extract structural details that were previously only accessible through low-energy experiments.

Beyond simply confirming the shape, the study provided the first experimental measurements of specific correlations between the protons and neutrons inside the nucleus. In a quantum system, the position of one particle is often linked to the position of another, a phenomenon known as correlation. The researchers were able to quantify these links for both pairs of particles and groups of three. They found that the arrangement of these particles creates a distinct signature in the collision debris. For instance, they measured how the overall size of the nucleus relates to its shape, finding a specific relationship that holds true for this isotope. These measurements serve as new benchmarks for theorists who are trying to build fundamental models of the nucleus from the ground up. By providing concrete numbers for these correlations, the study offers a new way to test and refine the theories that describe how the strong nuclear force binds matter together.

The implications of this work extend beyond the structure of a single atom. The ability to extract detailed nuclear structure from high-energy collisions opens a new avenue for studying the fundamental properties of matter. The researchers noted that their findings could help improve calculations for neutrinoless double-beta decay, a rare process that, if observed, would prove that neutrinos are their own antiparticles. Understanding the precise arrangement of nucleons in the parent nuclei is crucial for predicting the rate of this decay. Furthermore, the techniques developed here could be applied to other isotopes, potentially revealing new details about the nuclear force and the behavior of matter under extreme conditions. The study also touched upon the lead-208 nucleus, finding evidence of a different type of deformation that had been suggested by other experiments, further validating the method's sensitivity.

This research demonstrates that the Large Hadron Collider, often associated with the search for new particles and the origins of the universe, can also serve as a powerful microscope for the atomic nucleus. By treating the collision as a sudden probe that freezes the nuclear configuration, scientists have turned a destructive event into a source of precise structural information. The team's success in mapping the xenon-129 nucleus suggests that this approach can be used to explore the ground states of many other isotopes, providing a wealth of new data for nuclear physicists. The findings confirm that the complex, many-body dynamics of protons and neutrons leave a clear and measurable imprint on the debris of high-energy collisions, bridging the gap between the high-energy world of particle physics and the low-energy world of nuclear structure.