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EPJ A Highlight - Modern three-body forces make neutron stars collapse

Density profile of the collapsed state of 10000 neutrons in the X-Y-plane along the symmetry axis Z=0 (schematic illustration). Polarized neutrons, which interact through incorrect three-body forces, concentrate in small spheres separated by 0.9 fermi. © Dmitry K. Gridnev et al.

Nuclear systems ranging from light nuclei to massive neutron stars can be well described by nucleons interacting through two-body and three-body forces. From electrostatics we know that two identical uniformly charged spheres repel at any distance but the repulsion disappears when the spheres completely overlap. Similarly, in some modern expressions of nuclear three-body force it is assumed that the nuclear repulsion between the three nucleons is zero when they occupy the same position in space.

The authors provide a mathematical proof that such form of the three-body force leads to the collapse of large neutron systems: N neutrons form a bound system with the energy growing as N3 (the effect becomes visible for N > 10000). The density of such system is illustrated in Fig. 1.

Thus, in order to be compatible with our knowledge of neutron stars - where the constituents form dense nuclear matter with a finite energy per particle - modern expressions for three-body nuclear forces have to be carefully assessed regarding their strong repulsive core which should not vanish even when nucleon triples overlap.

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