Momentum sharing in imbalanced Fermi systems

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Science  31 Oct 2014:
Vol. 346, Issue 6209, pp. 614-617
DOI: 10.1126/science.1256785

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Scattering electrons off nuclear targets

Atomic nuclei consist of fermions—protons and neutrons—bound together by interactions. Because two identical fermions cannot occupy the same quantum state, both protons and neutrons have a broad range of momenta inside the nucleus. Hen et al. scattered electrons off nuclei of varying sizes to study the distribution of the protons' and neutrons' momenta. Protons formed high-momentum pairs with neutrons much more frequently than with other protons. Thus, surprisingly, the average momentum of a neutron was lower than that of a proton, even in nuclei with a larger number of neutrons than protons.

Science, this issue p. 614


The atomic nucleus is composed of two different kinds of fermions: protons and neutrons. If the protons and neutrons did not interact, the Pauli exclusion principle would force the majority of fermions (usually neutrons) to have a higher average momentum. Our high-energy electron-scattering measurements using 12C, 27Al, 56Fe, and 208Pb targets show that even in heavy, neutron-rich nuclei, short-range interactions between the fermions form correlated high-momentum neutron-proton pairs. Thus, in neutron-rich nuclei, protons have a greater probability than neutrons to have momentum greater than the Fermi momentum. This finding has implications ranging from nuclear few-body systems to neutron stars and may also be observable experimentally in two-spin–state, ultracold atomic gas systems.

  • The collaboration on this paper consists of all listed authors. There are no additional collaborators.

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