Quadrupole moments and proton-neutron structure in p-shell mirror nuclei

Electric quadrupole (E2) matrix elements provide a measure of nuclear deformation and related collective structure.  While the experimental electric quadrupole moment only measures the proton distribution, both proton and neutron quadrupole moments are needed to probe proton-neutron asymmetry in the nuclear deformation.  In ab initio calculations, converged no-core configuration interaction (NCCI) results for quadrupole moments are particularly challenging to obtain, due to sensitivity to long-range behavior of the wave functions.  However, we find that ratios of quadrupole moments (either across a mirror pair, or the proton/neutron moment ratio) are more robustly converged.  We compare with measured mirror ratios, explore how well mirror symmetry holds for calculated quadrupole moments, and interpret the results in terms of cluster and Elliott SU(3) descriptions.  A similar approach may be applied to other long-range observables, such as proton and neutron root-mean-square radii as indicators of halo structure.