First direct measurement of ${}^{48}$Ca single ${\hat{I}}^2$-decay Q value with the TITAN Penning trap

Abstract

Neutrinoless double ${\hat{I}}^2$-decay (0$\hat{I}½{\hat{I}}^2{\hat{I}}^2$), if observed, would provide unequivocal evidence of physics beyond the Standard Model. ${}^{48}$Ca is an interesting candidate system to study because it has the largest Q value among all 2${\hat{I}}^2$ transitions and is also unstable against single ${\hat{I}}^2$-decay. The observation of both ${\hat{I}}^2$ and 2${\hat{I}}^2$-decay in the same isotope would provide a unique opportunity to benchmark theoretical calculations of ${\hat{I}}^2$ and 2${\hat{I}}^2$-decay matrix elements and could provide insight on the quenching of the axial vector coupling constant, g${}_A$. We performed a precise measurement of the ${}^{48}$Ca ${\hat{I}}^2$-decay Q value using the TITAN Penning trap mass spectrometer at the TRIUMF facility. This was achieved through cyclotron frequency ratio measurements of ${}^{48}$Ca${}^{+}$/${}^{48}$Sc${}^{+}$ and ${}^{48}$Sc${}^{+}$/${}^{48}$Ti${}^{+}$ using the Time-of-Flight Ion Cyclotron Resonance technique. The ${}^{48}$Ca ${\hat{I}}^2$-decay Q value was determined to be 279.14(50) keV, a factor of 10 more precise than the previous value given in the 2020 Atomic Mass Evaluation [Chin. Phys. C 45, 030003 (2021)]. This Q value was used to determine the ${}^{48}$Ca ${\hat{I}}^2$-decay partial half-life, with the result $T_{1/2}^2$ = 5.09(5) x 10${}^{20}$ ($g_A^{-2}$) y. Our ${}^{48}$Ca ${\hat{I}}^2$-decay half-life was determined to a precision of 1%, a factor of 30 improvement compared to calculations with the previous Q value. Our result is marginally closer to the experimental lower limit $T_{1/2}^2$ > 1.1 x 10${}^{20}$ y, but still a factor 5 longer. It is also a factor of 10 longer than the observed 2$\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay mode with $T_{1/2}^{2\hat{I}½{\hat{I}}^2{\hat{I}}^2}=5.96_{-1.08}^{+1.39}$ x 10${}^{19}$ y. Hence, it could be possible to observe ${}^{48}$Ca ${\hat{I}}^2$-decay in future experiments, strengthening the potential importance of ${}^{48}$Ca to benchmark nuclear structure and 2${\hat{I}}^2$-decay studies.

7 pages, 6 figures