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

Abstract

Neutrinoless double $β$-decay (0$νββ$), 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$β$ transitions and is also unstable against single $β$-decay. The observation of both $β$ and 2$β$-decay in the same isotope would provide a unique opportunity to benchmark theoretical calculations of $β$ and 2$β$-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 $β$-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 $β$-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 $β$-decay partial half-life, with the result $T_{1/2}^β$ = 5.09(5) x 10$^{20}$ ($g_{A}^{-2}$) y. Our $^{48}$Ca $β$-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 $T_{1/2}^β$ > 1.1 x 10$^{20}$ y, but still a factor 5 longer. It is also a factor of 10 longer than the observed 2$νββ$ decay mode with $T_{1/2}^{2νββ} = 5.96^{+1.39}_{-1.08}$ x 10$^{19}$ y. Hence, it could be possible to observe $^{48}$Ca $β$-decay in future experiments, strengthening the potential importance of $^{48}$Ca to benchmark nuclear structure and 2$β$-decay studies.