Direct High-Precision Measurement of the Mass Difference of ^77 As– ^77 Se Related to Neutrino Mass Determination

The first direct determination of the ground-state-to-ground-state β ^- -decay Q-value of ^77 As to ^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 Se from γ -ray spectroscopy, the ground-state-to-excited-state Q-value of the transition ^77 As (3/2 ^- , ground state) → ^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 ^- -decay transition for neutrino mass determination is energetically allowed at a confidence level of about 60 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 years for this first-forbidden non-unique transition. Since the half-life of ^77 As is only ≈ 2 days, usage of it as source for rare-event experiments searching for the electron antineutrino mass would be challenging.