Ground-state mass of ²²Al and test of state-of-the-art ab initio calculations

The ground-state mass excess of the T-z = -2 drip-line nucleus Al-22 is measured for the first time as 18103(10)keV using the newly-developed B rho-defined isochronous mass spectrometry method at the cooler storage ring in Lanzhou. The new mass excess value allowed us to determine the excitation energies of the two low-lying 1(+) states in Al with significantly reduced uncertainties of 51 keV. When compared to the analogue states in its mirror nucleus F-22, the mirror energy differences of the two 1(+) states in the Al-22-F-22 mirror pair are determined to be -625(51) keV and -330(51)keV. The excitation energies and mirror energy differences are used to test the state-of-the-art ab initio valence-space in-medium similarity renormalization group calculations with four sets of interactions derived from the chiral effective field theory. The mechanism leading to the large mirror energy differences is investigated and attributed to the occupation of the pi s(1/2) orbital.