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Register a new userAnother look at the impact of an eV-mass sterile neutrino on the effective neutrino mass of neutrinoless double-beta decays
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The possible existence of an eV-mass sterile neutrino, slightly mixing with ordinary active neutrinos, is not yet excluded by neutrino oscillation experiments. Assuming neutrinos to be Majorana particles, we explore the impact of such a sterile neutrino on the effective neutrino mass of neutrinoless double-beta decays $langle m rangle^prime_{ee} equiv m^{}_1 |V^{}_{e1}|^2 e^{{rm i}rho} + m^{}_2 |V^{}_{e2}|^2 + m^{}_3 |V^{}_{e3}|^2 e^{{rm i}sigma} + m^{}_4 |V^{}_{e4}|^2 e^{{rm i}omega}$, where $m^{}_i$ and $V^{}_{ei}$ (for $i = 1, 2, 3, 4$) denote respectively the absolute masses and the first-row elements of the 4$times$4 neutrino flavor mixing matrix $V$, for which a full parametrization involves three Majorana-type CP-violating phases ${rho, sigma, omega}$. A zero effective neutrino mass $|langle m rangle^prime_{ee}| = 0$ is possible no matter whether three active neutrinos take the normal or inverted mass ordering, and its implications for the parameter space are examined in great detail. In particular, given the best-fit values of $m^{}_4 approx 1.3~{rm eV}$ and $|V^{}_{e4}|^2 approx 0.019$ from the latest global analysis of neutrino oscillation data, a three-dimensional view of $|langle m rangle^prime_{ee}|$ in the $(m^{}_1, rho)$-plane is presented and further compared with that of the counterpart $|langle m rangle^{}_{ee}|$ in the absence of any sterile neutrino.
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In this paper, we emphasize why it is important for future neutrinoless double-beta ($0 ubetabeta$) decay experiments to reach the sensitivity to the effective neutrino mass $|m^{}_{betabeta}| approx 1~{rm meV}$. Assuming such a sensitivity and the precisions on neutrino oscillation parameters after the JUNO experiment, we fully explore the constrained regions of the lightest neutrino mass $m^{}_1$ and two Majorana-type CP-violating phases ${rho, sigma}$. The implications for the neutrino mass spectrum, the effective neutrino mass $m^{}_beta$ in beta decays and the sum of three neutrino masses $Sigma equiv m^{}_1 + m^{}_2 + m^{}_3$ relevant for cosmological observations are also discussed.
The observation of neutrinoless double beta decay will have important consequences. First it will signal that lepton number is not conserved and the neutrinos are Majorana particles. Second, it represents our best hope for determining the absolute neutrino mass scale at the level of a few tens of meV. To achieve the last goal, however, certain hurdles have to be overcome involving particle, nuclear and experimental physics. Particle physics is important since it provides the mechanisms for neutrinoless double beta decay. In this review we emphasize the light neutrino mass mechanism. Nuclear physics is important for extracting the useful information from the data. One must accurately evaluate the relevant nuclear matrix elements, a formidable task. To this end, we review the recently developed sophisticated nuclear structure approaches, employing different methods and techniques of calculation. We also examine the question of quenching of the axial vector coupling constant, which may have important consequences on the size of the nuclear matrix elements. From an experimental point of view it is challenging, since the life times are extremely long and one has to fight against formidable backgrounds. One needs large isotopically enriched sources and detectors with good energy resolution and very low background.
Recent neutrino experiment results show a preference for the normal neutrino mass ordering. The global efforts to search for neutrinoless double beta decays undergo a broad gap with the approach to the prediction in the three-neutrino framework based on the normal ordering. This research is intended to show that it is possible to find a neutrinoless double beta decay signal even with normal ordered neutrino masses. We propose the existence of a light sterile neutrino as a solution to the higher effective mass of the electron neutrino expected by the current experiments. A few short-baseline oscillation experiments gave rise to a limit on the mass of the sterile neutrino and its mixing with the lightest neutrino. We demonstrate that the results of neutrinoless double beta decays can also narrow down the range of the mass and the mixing angle of the light sterile neutrino.
Assuming 3 neutrino mixing and massive Majorana neutrinos, we analyze the implications of the results of the solar neutrino experiments, including the latest SNO data, which favor the LMA MSW solution of the solar neutrino problem with tan^2 theta_sol < 1, for the predictions of the effective Majorana mass |<m>| in neutrinoless double beta decay. For cos (2 theta_sol) geq 0.26, which follows from the analysis of the new solar neutrino data, we find significant lower limits on |<m>| in the cases of quasi-degenerate and inverted hierarchy neutrino mass spectrum, |<m>| geq 0.035 eV and |<m>| geq 8.5 10^-3 eV, respectively. If the spectrum is hierarchical the upper limit holds |<m>| leq 8.2 10^-3 eV. Correspondingly, not only a measured value of |<m>| eq 0, but even an experimental upper limit on |<m>| of the order of few 10^-2 eV can provide information on the type of the neutrino mass spectrum; it can provide also a significant upper limit on the mass of the lightest neutrino m1. A measured value of |<m>| geq 0.2 eV, combined with data on neutrino masses from the tritium beta-decay experiment KATRIN might allow to establish whether the CP-symmetry is violated in the lepton sector.
We quantify the extent to which future experiments will test the existence of neutrinoless double-beta decay mediated by light neutrinos with inverted-ordered masses. While it remains difficult to compare measurements performed with different isotopes, we find that future searches will fully test the inverted ordering scenario, as a global, multi-isotope endeavor. They will also test other possible mechanisms driving the decay, including a large uncharted region of the allowed parameter space assuming that neutrino masses follow the normal ordering.
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