Low-energy limits on heavy Majorana neutrino masses from the neutrinoless double-beta decay and non-unitary neutrino mixing

Zhi-Zhong Xing

doi:10.1016/j.physletb.2009.07.040

Abstract

In the type-I seesaw mechanism, both the light Majorana neutrinos (ν1,ν2,ν3) and the heavy Majorana neutrinos (N1,…,Nn) can mediate the neutrinoless double-beta (0νββ) decay. We point out that the contribution of νi to this 0νββ process is also dependent on the masses Mk and the mixing parameters Rek of Nk as a direct consequence of the exact seesaw relation, and the effective mass term of νi is in most cases dominant over that of Ni. We obtain a new bound |∑Rek2Mk|<0.23 eV (or <0.85 eV as a more conservative limit) at the 2σ level, which is much stronger than |∑Rek2Mk−1|<5×10−8 GeV−1 used in some literature, from current experimental constraints on the 0νββ decay. Taking the minimal type-I seesaw scenario for example, we illustrate the possibility of determining or constraining two heavy Majorana neutrino masses by using more accurate low-energy data on lepton number violation and non-unitarity of neutrino mixing.

Highlights

Almost undebatable evidence for finite neutrino masses and large neutrino mixing angles has recently been achieved from solar, atmospheric, reactor and accelerator neutrino oscillation experiments [1,2,3,4]
The reason is that the contribution of νi to the 0νββ decay is in most cases dominant over the contribution of Nk to the same process, leading to a much stronger bound on Mk and Rek through the exact seesaw relation: n
We have carefully examined the contributions of both light Majorana neutrinos νi with masses mi and heavy Majorana neutrinos Nk with masses Mk to the 0νββ decay in the type-I seesaw mechanism, in which the light neutrino mixing matrix V is non-unitary due to the non-vanishing coupling matrix R between Nk and charged leptons

Summary

INTRODUCTION

Almost undebatable evidence for finite neutrino masses and large neutrino mixing angles has recently been achieved from solar, atmospheric, reactor and accelerator neutrino oscillation experiments [1,2,3,4]. The simplest way to generate non-zero but tiny neutrino masses mi for νi is to extend the SM by introducing at least two right-handed neutrinos and allowing lepton number violation In this well-known (type-I) seesaw mechanism [5], the SU(2)L × U(1)Y gauge-invariant mass terms of charged leptons and neutrinos are given by. If the TeV scale is really a fundamental scale, we are reasonably motivated to speculate that possible new physics existing at this scale and responsible for the electroweak symmetry breaking might be responsible for the origin of neutrino masses In this sense, it is meaningful to investigate the TeV seesaw mechanism and balance its theoretical naturalness and experimental testability at the energy frontier set by the LHC [7]. Taking the minimal type-I seesaw scenario [12] for example, we shall illustrate the possibility of determining or constraining two heavy Majorana neutrino masses by using more accurate low-energy data on the 0νββ decay and non-unitary neutrino mixing and CP violation

STRONGER BOUND ON THE 0νββ DECAY

THE MINIMAL SEESAW SCENARIO

SUMMARY