Suppressed electric quadrupole collectivity in Si32

Lying between 16O and 40Ca, the $sd$ shell is well described by robust phenomenological and ab initio nuclear theories. In this work, however, we highlight an unexplained reduction in electric-quadrupole strength in the rare isotope 32Si, studied through sub-barrier Coulomb excitation. It is found that the oblate nature of the deformation is well reproduced, while the absolute scale of quadrupole deformation, however, is inhibited by approximately a factor of 2 compared to theoretical predictions. Through comparison with shell-model and ab initio calculations, we present a number of possible explanations for this inhibited $E2$ strength. By comparing the results of these calculations to multiple observables, we conclude that there is a reduced role for out-of-space excitations in 32Si, resulting in a reduction in the corrections normally applied to both models.