Direct high-precision measurement of the mass difference of ⁷⁷As–⁷⁷Se related to neutrino mass determination

The first direct determination of the ground-state-to-ground-state β ⁻ β - -decay Q -value of ⁷⁷ 77 As to ⁷⁷ 77 Se was performed by measuring their atomic mass difference utilizing the double Penning trap mass spectrometer, JYFLTRAP. The resulting Q -value is 684.463(70) keV, representing a remarkable 24-fold improvement in precision compared to the value reported in the most recent Atomic Mass Evaluation (AME2020). With the significant reduction of the uncertainty of the ground-state-to-ground-state Q -value and knowledge of the excitation energies in ⁷⁷ 77 Se from γ γ -ray spectroscopy, the ground-state-to-excited-state Q -value of the transition ⁷⁷ 77 As (3/2 ⁻ - , ground state) → → ⁷⁷ 77 Se * ∗ (5/2 ⁺ + , 680.1035(17) keV) was refined to be 4.360(70) keV. We confirm that this potential low Q -value β ⁻ β - -decay transition for neutrino mass determination is energetically allowed at a confidence level of about 60 σ σ . Nuclear shell-model calculations with two well-established effective Hamiltonians were used to estimate the partial half-life for the low Q -value transition. The half-life was found to be of the order of 10 ⁹ 9 years for this first-forbidden non-unique transition. Since the half-life of ⁷⁷ 77 As is only ≈ ≈ 2 days, usage of it as source for rare-event experiments searching for the electron antineutrino mass would be challenging.