Elucidation of the anomalous A = 9 isospin quartet behaviour

TL;DR

This paper explains the anomalous behaviour of the A=9 isospin quartet by analyzing high-precision mass measurements and shell model calculations, revealing the role of isospin mixing with nearby T=1/2 states.

Contribution

It provides a new explanation for the A=9 quartet anomalies based on isospin mixing with nearby T=1/2 states, challenging previous Coulomb-based hypotheses.

Findings

Identification of a cubic term in the IMME for the 3/2- quartet.

Vanishing cubic term for the 1/2- multiplet.

Presence of T=1/2 states near the calculated energies.

Abstract

Recent high-precision mass measurements of $^{9}$Li and $^{9}$Be, performed with the TITAN Penning trap at the TRIUMF ISAC facility, are analyzed in light of state-of-the-art shell model calculations. We find an explanation for the anomalous Isobaric Mass Multiplet Equation (IMME) behaviour for the two $A$ = 9 quartets. The presence of a cubic $d$ = 6.3(17) keV term for the $J^{\pi}$ = 3/2$^{-}$ quartet and the vanishing cubic term for the excited $J^{\pi}$ = 1/2$^{-}$ multiplet depend upon the presence of a nearby $T$ = 1/2 state in $^{9}$B and $^{9}$Be that induces isospin mixing. This is contrary to previous hypotheses involving purely Coulomb and charge-dependent effects. $T$ = 1/2 states have been observed near the calculated energy, above the $T$ = 3/2 state. However an experimental confirmation of their $J^{\pi}$ is needed.