According to the shell model, the protons and neutrons—collectively known as nucleons—that make up nuclei move independently in discrete quantum orbits and are bound by an average potential created by their mutual attractive interactions. But that picture is too naive. In the 1980s, electron-scattering experiments that knocked protons from both valence and deeply bound nuclear orbitals found only 60–70% of the number predicted by the mean-field approximation.
At the time, some theorists attributed the difference to correlations between nucleons. A rich variety of low-energy nuclear phenomena, including collective rotations and vibrations, shape mixing, and superfluidity, are known to originate in correlations between nucleons separated by several femtometers. But those long-range correlations make up less than half the difference. Short-range correlations (SRCs), on the scale of a femtometer or less, can close the gap, but direct evidence for them has proven elusive. Still, physicists have surmised their presence for decades, not...
