Science 346, 614–617 (2014)

Credit: © AF ARCHIVE / ALAMY

Nucleons are fermions and, if not for their interaction, by the exclusion principle we would expect those in the majority — say, the neutrons — to have a higher average momentum. But short-range interactions lead to the exact opposite situation: when neutrons are abundant, high-momentum protons are more likely than high-momentum neutrons.

Using high-energy electron scattering, Or Hen and collaborators measured the momentum distributions of protons and neutrons in neutron-rich nuclei. They found that high-momentum neutron–proton pairs were much more common than proton–proton pairs. Invoking a ballroom analogy, the authors noted that if the ladies outnumber the gentlemen, the interactions involved in choosing dance partners will make for some unlucky ladies who do not get to dance. In neutron-rich atomic nuclei, high-momentum protons and neutrons will also pair up, leaving a larger number of low-momentum neutrons without a partner. This increases the average proton momentum leading to the situation in which the average momentum of a neutron is actually lower than that of a proton.

The authors note that further insight into this surprising conclusion may come from other interacting fermion systems, namely ultracold atomic gas experiments.