Neutrinoless Double Beta Decay: 2015 Review

Abstract

The discovery of neutrino masses through the observation of oscillations boosted the importance of neutrinoless double beta decay (0νββ). In this paper, we review the main features of this process, underlining its key role both from the experimental and theoretical point of view. In particular, we contextualize the 0νββ in the panorama of lepton-number violating processes, also assessing some possible particle physics mechanisms mediating the process. Since the 0νββ existence is correlated with neutrino masses, we also review the state-of-art of the theoretical understanding of neutrino masses. In the final part, the status of current 0νββ experiments is presented and the prospects for the future hunt for 0νββ are discussed. Also, experimental data coming from cosmological surveys are considered and their impact on 0νββ expectations is examined.

Global constraints on absolute neutrino masses and their ordering

“…Such limits are getting close to the mass scale |∆m 2 | 0.05 eV appearing in Eq. (3) where some sensitivity of cosmological data to mass ordering may be emerging [15]. Bounds on m β , although free from specific assumptions, are an order of magnitude weaker at present [1,14].…”

“…(5), G is the (calculable) phase space and M is the nuclear matrix element (NME) for a candidate nucleus [15,[50][51][52]. A worldwide search is underway to find possible 0νββ decay signatures in a variety of nuclei, and lower limits on the corresponding half lives have been placed [53][54][55].…”

“…In particular, three main observables can probe the absolute mass spectrum: (i) the effective neutrino mass m β in β decay; (ii) the effective mass m ββ in neutrinoless double beta (0νββ) decay, if neutrinos are Majorana fermions; and (iii) the total neutrino mass Σ in cosmology; see, e.g., the reviews in [13][14][15][16].…”

Breakings of the neutrino μ-τ reflection symmetry

“…Given small values of symmetry-breaking parameters (i.e., R 1 = 0.1, I 1 = 0.1, R 2 = 0.1 and I 2 = 0.1), the resulting ∆θ appears as: On the other hand, the resulting ∆δ turns out to be: Finally, we discuss the consequences of breaking of µ-τ reflection symmetry on the allowed range of effective Majorana neutrino mass |M ee | which directly controls the rates of neutrinoless double-beta decays [36][37][38][39] Because the symmetry-breaking parameter 3 = Im(M ee )/Re(M ee ) should be a small quantity (e.g., | 3 | ≤ 0.1) if we want to maintain the µ-τ reflection symmetry as an approximate one, the value of 20) can be well approximated by that of |Re(M ee )|. It is thus fair to say that the consequences of breaking of µ-τ reflection symmetry on the allowed range of |M ee | are negligibly small.…”

Constraining the right-handed gauge boson mass from lepton number violating meson decays in a low scale left-right model

“…The same feature is also applicable for a TeV scale MLRSM, where the right handed neutrino N together with a right handed gauge boson W R can give promising signals in 0νββ and collider searches [21][22][23][24][25][26][27][28]. In addition, W R can also be looked for, through dijet searches [29,30].…”