Understanding and predicting the formation of shell structure from nuclear forces is a central challenge for nuclear physics. While the magic numbers N = 2, 8, 20 are generally well understood, N = 28 is the first standard magic number that is not reproduced in microscopic theories with two-nucleon forces. In this paper, we show that three-nucleon forces give rise to repulsive interactions between two valence neutrons that are key to explain ~(48)Ca as a magic nucleus, with a high 2 ~+ excitation energy and a concentrated magnetic dipole transition strength. The repulsive three-nucleon mechanism improves the agreement with experimental binding energies.
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