In spite of the high-density and strongly correlated nature of the atomic nucleus, experimental and theoretical evidence suggests that around particular ‘magic’ numbers of nucleons, nuclear properties are governed by a single unpaired nucleon1,2. A microscopic understanding of the extent of this behavior and its evolution in neutron-rich nuclei remains an open question in nuclear physics3–5. The indium isotopes are considered a textbook example of this phenomenon6, where the constancy of their electromagnetic properties indicated a single unpaired proton hole can provide the identity of a complex many-nucleon system6,7.
Here, we present precision laser spectroscopy measurements performed to investigate the validity of this simple single-particle picture. Observation of an abrupt change in the dipole moment at N = 82 reveals that while the simple single-particle picture indeed dominates at neutron magic number N = 822,8, it does not for previously studied isotopes.
To investigate the microscopic origin of these observations, our work provides a combined effort with developments in two complementary nuclear many-body methods: ab-initio valence space in-medium similarity normalization group and density functional theory. We find that the inclusion of time symmetry-breaking mean fields is essential for a correct description of nuclear magnetic properties, which were previously poorly constrained. These experimental and theoretical findings are key to understand how seemingly simple single-particle phenomena naturally emerge from complex interactions among protons and neutrons, including from ab-initio nuclear theories.