Tentative sensitivity of future 0\u03bd\u03b2\u03b2-decay experiments to neutrino masses and Majorana CP phases

Guo-Yuan Huang,Shun Zhou

doi:10.1007/jhep03(2021)084

Abstract

In the near future, the neutrinoless double-beta (0νββ) decay experiments will hopefully reach the sensitivity of a few meV to the effective neutrino mass |mββ|. In this paper, we tentatively examine the sensitivity of future 0νββ-decay experiments to neutrino masses and Majorana CP phases by following the Bayesian statistical approach. Provided experimental setups corresponding to the experimental sensitivity of |mββ| ≃ 1 meV, the null observation of 0νββ decays in the case of normal neutrino mass ordering leads to a very competitive bound on the lightest neutrino mass m1. Namely, the 95% credible interval in the Bayesian approach turns out to be 1.6 meV ≲ m1 ≲ 7.3 meV or 0.3 meV ≲ m1 ≲ 5.6 meV when the uniform prior on m1/eV or on log10(m1/eV) is adopted. Moreover, one of two Majorana CP phases is strictly constrained, i.e., 140° ≲ ρ ≲ 220° for both scenarios of prior distributions of m1. In contrast, if a relatively worse experimental sensitivity of |mββ| ≃ 10 meV is assumed, the constraint on the lightest neutrino mass becomes accordingly 0.6 meV ≲ m1 ≲ 26 meV or 0 ≲ m1 ≲ 6.1 meV, while two Majorana CP phases will be essentially unconstrained. In the same statistical framework, the prospects for the determination of neutrino mass ordering and the discrimination between Majorana and Dirac nature of massive neutrinos in the 0νββ-decay experiments are also discussed. Given the experimental sensitivity of |mββ| ≃ 10 meV (or 1 meV), the strength of evidence to exclude the Majorana nature under the null observation of 0νββ decays is found to be inconclusive (or strong), no matter which of two priors on m1 is taken.

Highlights

2δ have been redefined as in ref. [4]
The upper bound on the absolute scale of neutrino masses extracted from the tritium betadecay experiments is mβ < 2.3 eV at the 95% confidence level (CL) from Mainz [10], mβ < 2.2 eV at the 95% CL from Troitsk [11], and mβ < 1.1 eV at the 90% CL from KATRIN [12], where the effective neutrino mass mβ for beta decays is defined as mβ ≡ m21|Ue1|2 + m22|Ue2|2 + m23|Ue3|2 1/2 with the moduli of the matrix elements of lepton flavor mixing matrix being |Ue1| = cos θ13 cos θ12, |Ue2| = cos θ13 sin θ12 and |Ue3| = sin θ13 in the standard parametrization
A robust statistical analysis is desirable to answer the following question: (i) given an experimental setup, what can we learn from a null signal after systematically taking into account the uncertainties of oscillation data, the nuclear matrix element and the phase-space factor? (ii) or to derive competitive bounds on the neutrino mass and Majorana phases, which kind of experimental setups will be required in the future? the latest global-fit analysis of neutrino oscillation data yields a 2σ hint at the normal neutrino mass ordering [26], so it is timely to investigate the physics potential of the future 0νββ-decay experiments that aim at the ultimate discovery even in the NO case

Summary

Introduction

2δ have been redefined as in ref. [4]. The other neutrino mixing angle θ23 is irrelevant for 0νββ decays. We tentatively examine the sensitivity of future 0νββ-decay experiments to neutrino masses and Majorana CP phases by following the Bayesian statistical approach.

Results

Conclusion