High-momentum nucleon pairs challenge the nuclear shell model

According to quantum mechanics, protons and neutrons in a nucleus must occupy well-defined and independent energy levels, or shells. That view guides the nuclear shell model, which can account for the spin of nuclear ground states, the stability of specific nuclei, and other properties. Despite those successes, the model doesn’t always agree with observations. Electron-scattering experiments on the nuclei of several elements record fewer protons in each shell than the nuclear shell model predicts. That mismatch between theory and observations may now be solved because of new measurements carried out by MIT’s Or Hen and the CLAS collaboration at the Thomas Jefferson National Accelerator Facility in Virginia.

As illustrated here, electrons from a 5.014 GeV beam slam into a target nucleus of deuterium, carbon, aluminum, iron, or lead to knock out individual nucleons. To reconstruct each nucleon’s initial momentum and relative abundance, Hen and colleagues measured the trajectory of ejected protons through a magnetic field. Using an electromagnetic calorimeter, they were also able to measure, for the first time, the interaction time of ejected neutrons. As predicted by the nuclear shell model, nuclei with more neutrons had a higher ratio of neutrons to protons, but only for nucleons with low initial momentum. Nucleons with high momentum, unaccounted for in the nuclear shell model, showed no trend. A phenomenon called n–p (neutron–proton) dominance may explain why: In the crowded nuclear environment, some high-momentum neutrons and protons pair up. When Hen and colleagues deduced the ratio of high-momentum to low-momentum protons, they found a 50% increase connected to nuclei with more neutrons. Previous experiments that did not measure n–p pairs would therefore have undercounted protons. The phenomenon of n–p dominance may also affect neutron-rich neutron stars, which, despite their name, contain some protons. If high-momentum protons pair up with neutrons, they may contribute disproportionately to such stars’ equation of state and cooling history. (M. Duer et al., CLAS collaboration, Nature, 2018, doi:10.1038/s41586-018-0400-z .)