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In atomic nuclei, protons and neutrons share energy and momentum in tight quarters. But exactly how they share the energy that keeps them bound within the nucleus—and even where they are within the nucleus—remain key puzzles for nuclear physicists.

A new study by researchers at Lawrence Livermore National Laboratory (LLNL) and Washington University in St. Louis tackled these questions by leveraging data from nuclear scattering experiments to make stringent constraints on how nucleons (neutrons and protons) arrange themselves in the nucleus. The research appears in two corresponding papers in Physical Review C and Physical Review Letters.

Their analysis shows that for several cornerstone nuclei, a tiny fraction of the protons and neutrons possess the lion's share of the overall energy that keeps them bound in nuclei, roughly 50 percent more than expected from standard theoretical treatments.

Further, the study makes new predictions for the "neutron skin" (a region where extra neutrons pile up) of several neutron-rich nuclei. In turn, these predictions are tightly connected to how large neutron stars grow and what elements are likely synthesized in neutron star mergers.