Z = 14 Magicity Revealed by the Mass of the Proton Dripline Nucleus Si 22

Using the Bρ-defined isochronous mass spectrometry technique, we conducted the first mass measurement of the proton dripline nucleus ^{22}Si. We confirm that ^{22}Si is bound against particle emission with S_{p}/S_{2p}=+1412(114)/+229(54) keV, fixing the proton dripline location for the Si element. By analyzing the mass differences of the neighboring sd-shell nuclei, we find that ^{22}Si exhibits a doubly magic character similar to its mirror partner ^{22}O, and that the mirror energy difference of ^{22}Si-^{22}O deviates from the predictions assuming mirror symmetry. Gamow shell-model calculations reveal that the average occupations of valence protons in ^{22}Si are nearly identical to those of valence neutrons in ^{22}O, supporting the Z=14 magicity in ^{22}Si. The observed mirror-symmetry breaking is attributed to the extended proton distribution in ^{22}Si arising from a small contribution of the unbound π2s_{1/2} orbital.