Towards the meV limit of the effective neutrino mass in neutrinoless double-beta decays

Abstract

We emphasize that it is extremely important for future neutrinoless double-beta (0vββ) decay experiments to reach the sensitivity to the effective neutrino mass |mββ| ≈ 1 meV. With such a sensitivity, it is highly possible to discover the signals of 0vp? decays. If no signal is observed at this sensitivity level, then either neutrinos are Dirac particles or stringent constraints can be placed on their Majorana masses. In this paper, assuming the sensitivity of |mββ| ≈ 1 meV for future 0vββ decay experiments and the precisions on neutrino oscillation parameters after the JUNO experiment, we fully explore the constrained regions of the lightest neutrino mass mi and two Majorana-type CP-violating phases {ρ,σ}. Several important conclusions in the case of normal neutrino mass ordering can be made. First, the lightest neutrino mass is severely constrained to a narrow range m1 ∈ [0.7,8] meV, which together with the precision measurements of neutrino mass-squared differences from oscillation experiments completely determines the neutrino mass spectrum m2 ∈ [8.6, 11.7] meV and m3 ∈ [50.3,50.9] meV. Second, one of the two Majorana CP-violating phases is limited to ρ ∈ [130°,230°], which cannot be obtained from any other realistic experiments. Third, the sum of three neutrino masses is found to be Σ ≡ m1 + m2 + m3 ∈ [59.2,72.6] meV, while the effective neutrino mass for beta decays turns out to be mβ = (|Ue1|2m2 + |Ue2|2m22 + |Ue3|2m2)1/2 ∈ [8.9,12.6] meV. These observations clearly set up the roadmap for future non-oscillation neutrino experiments aiming to solve the fundamental problems in neutrino physics.