Phenomenological advantages of the normal neutrino mass ordering *

Abstract

The preference of the normal neutrino mass ordering from the recent cosmological constraint and the global fit of neutrino oscillation experiments does not seem like a wise choice at first glance since it obscures the neutrinoless double beta decay and hence the Majorana nature of neutrinos. Contrary to this naive expectation, we point out that the actual situation is the opposite. The normal neutrino mass ordering opens the possibility of excluding the higher solar octant and simultaneously measuring the two Majorana CP phases in future experiments. Especially, the funnel region will completely disappear if the solar mixing angle takes the higher octant. The combined precision measurement by the JUNO and Daya Bay experiments can significantly reduce the uncertainty in excluding the higher octant. With a typical sensitivity on the effective mass , the neutrinoless double beta decay experiment can tell if the funnel region really exists and hence exclude the higher solar octant. With the sensitivity further improved to sub-meV, the two Majorana CP phases can be simultaneously determined. Thus, the normal neutrino mass ordering clearly shows phenomenological advantages over the inverted one.

Inference of neutrino nature and Majorana CP phases from $$\mathbf{0}{\nu \beta \beta }$$ decays with inverted mass ordering

“…As the overall CP phase of m ee is non-physical, it is a convenient choice to assign two Majorana CP phases to the lightest and the heaviest neutrino states [9][10][11], such that only one phase parameter will survive when the lightest neutrino mass is vanishing. The importance of 0νββ decay searches for neutrino physics is well recognized [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], as it can answer: (i) the nature of neutrinos, (ii) the Majorana CP phases, (iii) the absolute scale of neutrino masses, and (iv) the neutrino mass ordering. While the neutrino masses can be better probed by other observations (e.g., cosmology and oscillation experiments), the 0νββ decay search seems to be the only reliable way towards the neutrino nature and the Majorana CP phases.…”

“…In the assumption of NO, a meV sensitivity to |m ee | is required to extract possible information of Majorana CP phases [14,18,19,[22][23][24][25], which is, however, far beyond the capability of the next-generation experiments in the near future. One would wish that IO is true from a practical point of view, as the full parameter space of IO will be covered with a 10 meV sensitivity in the not-too-distant future.…”

Tentative sensitivity of future 0νββ-decay experiments to neutrino masses and Majorana CP phases

“…where G 0ν denotes the relevant phase-space factor, M 0ν is the nuclear matrix element (NME), and m e = 0.511 MeV is the electron mass. As advocated by Particle Data Group [3], the lepton flavor mixing matrix U is usually parametrized in terms of three mixing angles {θ 12 , θ 13 , θ 23 2δ have been redefined as in ref. [4].…”

“…Second, if massive neutrinos are indeed Majorana particles, then two associated CP-violating phases {ρ, σ} are fundamental parameters in nature and must be experimentally determined. At present, the 0νββ decays are the unique feasible pathway to get close to this goal [17][18][19][20][21][22][23]. In this connection, the neutrino-antineutrino oscillations and other lepton-number-violating processes could in principle also provide some useful information about Majorana CP phases [24,25], but the observations of these processes are currently still far away from reality.…”

“…In this connection, the neutrino-antineutrino oscillations and other lepton-number-violating processes could in principle also provide some useful information about Majorana CP phases [24,25], but the observations of these processes are currently still far away from reality. Even though a number of analytical studies of the effective neutrino mass |m ββ | have been performed in the literature [17][18][19][20][21][22][23], some particular values of |m ββ | are assumed to derive the constraints on neutrino masses and Majorana CP phases. However, the effective neutrino mass |m ββ | itself is not a direct observable of 0νββ-decay experiments.…”