Nuclear structure of dripline nuclei elucidated through precision mass measurements of $^{23}$Si, $^{26}$P, $^{27,28}$S, and $^{31}$Ar

TL;DR

This study uses advanced mass spectrometry to precisely measure the masses of certain proton-rich isotopes, confirming their bound states, locating the proton dripline, and revealing mirror symmetry breaking linked to extended charge distributions.

Contribution

First mass measurements of $^{23}$Si, $^{26}$P, $^{27}$S, and $^{31}$Ar, and improved data for $^{28}$S, elucidating nuclear structure at the dripline and explaining mirror symmetry deviations.

Findings

Confirmed isotopes are bound and located the proton dripline.

Observed significant deviations in mirror energy differences from mirror symmetry predictions.

Identified extended charge distributions as a cause for mirror-symmetry breaking.

Abstract

Using the B$\rho$-defined isochronous mass spectrometry technique, we report the first determination of the $^{23}$Si, $^{26}$P, $^{27}$S, and $^{31}$Ar masses and improve the precision of the $^{28}$S mass by a factor of 11. Our measurements confirm that these isotopes are bound and fix the location of the proton dripline in P, S, and Ar. We find that the mirror energy differences of the mirror-nuclei pairs $^{26}$P-$^{26}$Na, $^{27}$P-$^{27}$Mg, $^{27}$S-$^{27}$Na, $^{28}$S-$^{28}$Mg, and $^{31}$Ar-$^{31}$Al deviate significantly from the values predicted assuming mirror symmetry. In addition, we observe similar anomalies in the excited states, but not in the ground states, of the mirror-nuclei pairs $^{22}$Al-$^{22}$F and $^{23}$Al-$^{23}$Ne. Using $ab~ initio$ VS-IMSRG and mean field calculations, we show that such a mirror-symmetry breaking phenomeon can be explained by the extended charge distributions of weakly-bound, proton-rich nuclei. When observed, this phenomenon serves as a unique signature that can be valuable for identifying proton-halo candidates.