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

Using the Bρ-defined isochronous mass spectrometry technique, we conducted the first mass measurement of the proton dripline nucleus ²²Si. We confirm that ²²Si is bound against particle emission with Sₚ/S₂ₚ=+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 ²²Si exhibits a doubly-magic character similar to its mirror partner ²²O, and that the mirror energy difference of ²²Si-²²O deviates from the predictions assuming mirror symmetry. Gamow shell-model calculations reveal that the average occupations of valence protons in ²²Si are nearly identical to those of valence neutrons in ²²O, supporting the $Z=14$ magicity in ²²Si. The observed mirror-symmetry breaking is attributed to the extended proton distribution in ²²Si arising from a small contribution of the unbound π2s1/2 orbital.