The shape of the xenon nucleus is no longer a mystery express
Atomic nuclei are not always like billiard balls. Some are elongated, like cigars, others are flattened. But xenon-129 resembles a potato — a bumpy triaxial ellipsoid. To determine such a shape, physicists used a method reminiscent of colliding two lumps of dough: accelerating nuclei to near-light speeds and smashing them, they reconstruct the original geometry from the scattering of fragments.
Using data from the Large Hadron Collider and clever mathematical methods to account for uncertainties, the team found that the xenon nucleus is stretched almost maximally in three directions at once. This unexpectedly matched quantum calculations, confirming the correctness of the Standard Model. The most amazing thing is that such a nucleus does not just rest, but constantly tumbles, rotating around several axes, like a confused spinning top. This rotation produces a bizarre radiation pattern that can now be captured in experiments.
Thus, the collider becomes a kind of spectroscope, allowing a glimpse into the heart of the atom. The increase in disorder (entropy) during collisions is inevitable, but it's the price for unique information. The idea goes back to Rutherford and his followers Fermi and Bethe.
Original abstract
as written by the authors arXiv:2606.03993
Imaging a quantum many-body system requires probes that resolve the coordinates of its constituents in sufficiently large event samples, allowing measurements of correlation functions [1-4]. High-energy nuclear collisions provide this opportunity on the nuclear scale [5], enabling features of colliding ions, such as their deformation, to be probed through particle correlation observables [6, 7]. However, a quantitative extraction of the correlation properties of nuclei from these measurements is still lacking. Here we show that this is possible for the nucleus $$^{129}$$Xe using Bayesian inference methods. We combine a deformed-rotor description of the colliding nuclei, which encodes the many-body dynamics of constituent neutrons and protons, with hydrodynamic simulations of the ensuing collision evolution. From a combined global analysis of Large Hadron Collider data on Xe-Xe and Pb-Pb collisions, we then infer that the shape of $$^{129}$$Xe is nearly maximally triaxial, which aligns with mean-field results for xenon isotopes away from shell closure [8, 9]. From this we evaluate two- and three-particle correlations in the nuclear ground state to provide new constraints for \textit{ab initio} methods in nuclear theory. We establish thus collider experiments as a means of quantifying correlations of protons and neutrons arising from residual forces of quantum chromodynamics.
Triaxial nuclei, like quantum spinning tops, tumble in space, rotating around several axes at once. This creates an intricate pattern of their radiation.