Direct determination of the atomic mass difference of the pairs $^{76}$As-$^{76}$Se and $^{155}$Tb-$^{155}$Gd rules out $^{76}$As and $^{155}$Tb as possible candidates for electron (anti)neutrino mass measurements

TL;DR

This study precisely measured the atomic mass differences of specific decay pairs using advanced Penning trap techniques, conclusively ruling out certain isotopes as candidates for electron neutrino mass experiments due to their forbidden decay transitions.

Contribution

The paper provides the first direct, highly precise measurements of the $Q$ values for $^{76}$As and $^{155}$Tb decays, significantly improving accuracy and excluding these isotopes from neutrino mass research.

Findings

Measured $Q$ values with 12 and 57 times higher precision.

Confirmed decay transitions are energetically forbidden at over 4$\sigma$.

Excluded $^{76}$As and $^{155}$Tb as candidates for low-$Q$-value neutrino mass experiments.

Abstract

The first direct determination of the ground-state-to-ground-state $Q$ values of the $\beta^-$ decay $^{76}$As $\rightarrow$ $^{76}$Se and the electron-capture decay $^{155}$Tb $\rightarrow$ $^{155}$Gd was performed utilizing the double Penning trap mass spectrometer JYFLTRAP. By measuring the atomic mass difference of the decay pairs via the phase-imaging ion-cyclotron-resonance (PI-ICR) technique, the $Q$ values of $^{76}$As $\rightarrow$ $^{76}$Se and $^{155}$Tb $\rightarrow$ $^{155}$Gd were determined to be 2959.265(74) keV and 814.94(18) keV, respectively. The precision was increased relative to earlier measurements by factors of 12 and 57, respectively. The new $Q$ values are 1.33 keV and 5 keV lower compared to the values adopted in the most recent Atomic Mass Evaluation 2020. With the newly determined ground-state-to-ground-state $Q$ values combined with the excitation energy from $\gamma$-ray spectroscopy, the $Q$ values for ground-state-to-excited-state transitions $^{76}$As (ground state) $\rightarrow$ $^{76}$Se$^*$ (2968.4(7) keV) and $^{155}$Tb (ground state) $\rightarrow$ $^{155}$Gd$^*$ (815.731(3) keV) were derived to be -9.13(70) keV and -0.79(18) keV. Thus we have confirmed that both of the $\beta^{-}$-decay and EC-decay candidate transitions are energetically forbidden at a level of at least 4$\sigma$, thus definitely excluding these two cases from the list of potential candidates for the search of low-$Q$-value $\beta^-$ or EC decays to determine the electron-(anti)neutrino mass.