Precision mass measurements of magnesium isotopes and implications for the validity of the isobaric mass multiplet equation

If the mass excess of neutron-deficient nuclei and their neutron-rich mirror partners are both known, it can be shown that deviations of the isobaric mass multiplet equation (IMME) in the form of a cubic term can be probed. Such a cubic term was probed by using the atomic mass of neutron-rich magnesium isotopes measured using the TITAN Penning trap and the recently measured proton-separation energies of Cl-29 and Ar-30. The atomic mass of Mg-27 was found to be within 1.6 sigma of the value stated in the Atomic Mass Evaluation. The atomic masses of Mg-28,Mg-29 were measured to be both within 1 sigma, while being 7 and 33 times more precise, respectively. Using the Mg-29 mass excess and previous measurements of Cl-29, we uncovered a cubic coefficient of d = 28(7) keV, which is the largest known cubic coefficient of the IMME. This departure, however, could also be caused by experimental data with unknown systematic errors. Hence there is a need to confirm the mass excess of S-28 and the one-neutron separation energy of Cl-29, which have both come from a single measurement. Finally, our results were compared with ab initio calculations from the valence-space in-medium similarity renormalization group, resulting in a good agreement.