Suppressed electric quadrupole collectivity in 32Si

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.