Collisions hint that four neutrons form a transient isolated entity

Lee G. Sobotka is in the Department of Physics and the McDonnell Center for the Space Sciences, Washington University in St. Louis, St Louis, Missouri 63130, USA, and in the Department of Chemistry, Washington University in St. Louis.

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Maria Piarulli is in the Department of Physics and the McDonnell Center for the Space Sciences, Washington University in St. Louis, St Louis, Missouri 63130, USA.

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To understand the nature of cohesion in any system, you have to push the system until it breaks. In the case of atomic nuclei, this means knowing when the short-range attractive nuclear force that acts between nucleons (protons and neutrons) surrenders to the repulsive long-range Coulomb force between protons, and the extent to which extreme proton-to-neutron ratios can exist in nature. Neutron stars are proof that huge gravitational forces can bind almost purely neutron matter, but whether the nuclear force can do so alone has been a long-standing question. In a paper in Nature, Duer et al.1 ‘break’ a helium-8 nucleus in search of an answer to this question, and find evidence hinting at the existence of a transient system comprising only four neutrons.

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