The 10−3 eV frontier in neutrinoless double beta decay

Abstract

The observation of neutrinoless double beta decay would allow to establish lepton number violation and the Majorana nature of neutrinos. The rate of this process in the case of 3-neutrino mixing is controlled by the neutrinoless double beta decay effective Majorana mass | m |. For a neutrino mass spectrum with normal ordering, which is favoured over the spectrum with inverted ordering by recent global fits, | m | can be significantly suppressed. Taking into account updated data on the neutrino oscillation parameters, we investigate the conditions under which | m | in the case of spectrum with normal ordering exceeds 10 −3 (5 × 10 −3 ) eV: | m |NO > 10 −3 (5 × 10 −3 ) eV. We analyse first the generic case with unconstrained leptonic CP violation Majorana phases. We show, in particular, that if the sum of neutrino masses is found to satisfy Σ > 0.10 eV, then | m |NO > 5 × 10 −3 eV for any values of the Majorana phases. We consider also cases where the values for these phases are either CP conserving or are in line with predictive schemes combining flavour and generalised CP symmetries.

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

“…(6) is about twice larger than the second term, the dominant uncertainty on the lower boundary comes from sin 2 θ 12 and ∆m 2 21 . For m 1 ∆m 2 21 8.6 meV and thus m 1 m 2 , we obtain |m ββ | L cos 2 θ 13 cos 2θ 12 ∆m 2 21 + m 2 1 − sin 2 θ 13 ∆m 2 31 + m 2 1 , so the uncertainty from sin 2 θ 12 continues to be dominant as clearly shown in Fig. 1.…”

“…1 with that in the right panel, as the improvements on sin 2 θ 12 and ∆m 2 21 after the JUNO measurements are remarkable. When the lightest neutrino mass m 1 increases, in particular for m 1 ∆m 2 21 8.6 meV, we obtain m 1 m 2 and the first term on the right-hand side of Eq. (5) is approximately given by m 1 cos 2 θ 13 , indicating that the dependence on the sin 2 θ 12 and ∆m 2 21 becomes very weak.…”

“…In the NO case, given two neutrino mass-squared differences ∆m 2 21 7.39 × 10 −5 eV 2 and ∆m 2 31 2.523 × 10 −3 eV 2 , we have m 3 = m 2 1 + ∆m 2 31 and m 2 = m 2 1 + ∆m 2 21 . The upper boundary is determined by Figure 1: The effective neutrino mass |m ββ | is shown as a function of the lightest neutrino mass m 1 in the NO case, where the gray region is allowed and the dashed curves refer to its boundaries.…”

“…In the literature, it has been noticed [17][18][19][20][21] that |m ββ | ≈ 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. The present study differs from previous works in two aspects.…”

Constraining the Effective Mass of Majorana Neutrino with Sterile Neutrino Mass for Inverted Ordering Spectrum

“…While for the Normal Ordering configuration of the mass spectrum the lower bound is |m ββ | N O << 10 −3 eV [18] and it can be extremely small depending on the values of the Dirac, Majorana phases and of the smallest neutrino mass. A condition is established using the recent global data on the neutrino oscillation parameters with NO(Normal Oredering) spectrum [22], which suggest |m ββ | must exceed 10 −3 (5 × 10 −3 ) eV. Neutrino double beta decay experiments are trying to cover the range of |m ββ | from the top and the current reachable upper limit reported by KamLAND-Zen collaboration is |m ββ | < (0.061 − 0.165)eV [16].…”