Abstract
We emphasize that it is extremely important for future neutrinoless double-beta ( ) decay experiments to reach the sensitivity to the effective neutrino mass . With such a sensitivity, it is highly possible to discover the signals of 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 for future decay experiments and the precisions on neutrino oscillation parameters after the JUNO experiment, we fully explore the constrained regions of the lightest neutrino mass and two Majorana-type CP-violating phases . 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 , which together with the precision measurements of neutrino mass-squared differences from oscillation experiments completely determines the neutrino mass spectrum and . Second, one of the two Majorana CP-violating phases is limited to , which cannot be obtained from any other realistic experiments. Third, the sum of three neutrino masses is found to be , while the effective neutrino mass for beta decays turns out to be . These observations clearly set up the roadmap for future non-oscillation neutrino experiments aiming to solve the fundamental problems in neutrino physics.
Highlights
Neutrino oscillation experiments have firmly established that neutrinos are massive particles and lepton flavors are significantly mixed [1]
We have explained why it is important for the future 0νββ decay experiments to reach the sensitivity of |mββ| ≈ 1 meV. With such a high sensitivity, one can place restrictive constraints on the lightest neutrino mass 0.7 meV ≤ m1 ≤ 8 meV and one of the Majorana CP phases 130◦ ≤ ρ ≤ 230◦. These constraints further imply that neutrino mass spectrum is almost fixed, namely, m1 ∈ [0.7, 8] meV, m2 ∈ [8.6, 11.7] meV and m3 ∈ [50.3, 50.9] meV, the effective neutrino mass for beta decays should be lying in the range 8.9 meV ≤ mβ ≤ 12.6 meV and the sum of three neutrino masses must be 59.2 meV ≤ Σ ≤ 72.6 meV
Among all the current 0νββ decay experiments in operation [27,28,29,30,31,32,33], the KamLAND-Zen collaboration has reported the best sensitivity |mββ| < (61 · · · 165) meV depending on the nuclear matrix element (NME) for the 0νββ decays of 136Xe
Summary
Neutrino oscillation experiments have firmly established that neutrinos are massive particles and lepton flavors are significantly mixed [1]. [16] is further significantly reduced, e.g., by two orders of magnitude, the ultimate sensitivity will hopefully be close to |mββ| ≈ 1 meV In the literature, it has been noticed [17,18,19,20,21] that |mββ| ≈ 1 meV could be set as a target value and useful information on the absolute neutrino masses and the Majorana CP phases can be obtained. We concentrate on the effective neutrino mass |mββ| in the NO case and update its value with both the latest global-fit results of all the relevant neutrino oscillation parameters and the future measurements from neutrino oscillation experiments With these input, it becomes clearer how much and definite the effective mass |mββ| ≈ 1 meV can tell us the information about the lightest neutrino mass m1 and the Majorana phases {ρ, σ}.
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