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 Cl29 and Ar30. The atomic mass of Mg27 was found to be within 1.6σ of the value stated in the Atomic Mass Evaluation. The atomic masses of Mg28,29 were measured to be both within 1σ, while being 7 and 33 times more precise, respectively. Using the Mg29 mass excess and previous measurements of Cl29, 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 S28 and the one-neutron separation energy of Cl29, 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.