Probing new physics in the neutrinoless double beta decay using electron angular correlation

Abstract

The angular correlation of the electrons emitted in the neutrinoless double beta decay (0ν2β) is presented using a general Lorentz invariant effective Lagrangian for the leptonic and hadronic charged weak currents. We show that the coefficient K in the angular correlation dΓ/d cos θ ∝ (1 − K cos θ) is essentially independent of the nuclear matrix element models and present its numerical values for the five nuclei of interest ( 76 Ge, 82 Se, 100 Mo, 130 Te, and 136 Xe), assuming that the 0ν2β-decays in these nuclei are induced solely by a light Majorana neutrino, νM . This coefficient varies between K = 0.81 (for the 76 Ge nucleus) and K = 0.88 (for the 82 Se and 100 Mo nuclei), calculated taking into account the effects from the nucleon recoil, the S and P -waves for the outgoing electrons and the electron mass. Deviation of K from its values derived here would indicate the presence of New Physics (NP) in addition to a light Majorana neutrino, and we work out the angular coefficients in several νM + NP scenarios for the 76 Ge nucleus. As an illustration of the correlations among the 0ν2β observables (half-life T 1/2 , the coefficient K, and the effective Majorana neutrino mass | m |) and the parameters of the underlying NP model, we analyze the left-right symmetric models, taking into account current phenomenological bounds on the right-handed WR-boson mass and the left-right mixing parameter ζ.It is now established beyond any doubt that the observed neutrinos have tiny but non-zero masses and they mix with each other, with both of these features following from the observation of the atmospheric and solar neutrino oscillations and from the long baseline neutrino oscillation experiments [1]. Theoretically, it is largely anticipated that the neutrinos are Majorana particles. Experimental evidence for the neutrinoless double beta decay (0ν2β) would deliver a conclusive confirmation of the Majorana nature of neutrinos, establishing the existence of physics beyond the standard model. This is the overriding interest in carrying out these experiments and in the related phenomenology [2].We recall that 0ν2β-decays are forbidden in the standard model (SM) by lepton number (LN) conservation, which is a consequence of the renormalizability of the SM. However, being the low energy limit of a more general theory, an extended version of the SM could contain nonrenormalizable terms (tiny to be compatible with experiments), in particular, terms that violate LN and allow the 0ν2β decay. Probable mechanisms of LN violation may include exchanges by: Majorana neutrinos ν M s [3, 4, 5] (the preferred mechanism after the observation of neutrino oscillations [1]), SUSY particles [6,7,8,9,10,11], scalar bilinears (SBs) [12], e.g. doubly charged dileptons (the component ξ −− of the SU (2) L triplet Higgs scalar etc.), leptoquarks (LQs) [13], right-handed W R bosons [5,14] etc. From these particles light νs are much lighter than the electron and others are much heavier than the proton. Therefore, there are two possible classes of mechanis...

Neutrinoless double-beta decay and physics beyond the standard model

“…More directly, it is also possible to infer the dominant mechanism purely within the context of neutrinoless double beta decay and closely related processes. Relevant techniques discussed in the literature include (i) the comparison of 0νββ decay rates in different isotopes [104][105][106] (this possibility is discussed in detail in the contribution by Fogli et al), (ii) the comparison of 0νβ − β − with 0νβ + β + [107], (iii) the comparison of 0νββ with electron capture [107], (iv) the comparison of of 0νββ decay to the ground state and an excited state [108] and (v) the experimental determination of the angular and energy distribution of the outgoing electrons in 0νββ [4,[109][110][111][112][113][114].…”

Probing new physics models of neutrinoless double beta decay with SuperNEMO

“…Such analyses would compare results for 0νββ with other experimental searches, but there are also ways to gain more information within the realm of 0νββ decay and related nuclear processes. Possible techniques include the analysis of angular and energy correlations between the electrons emitted in the 0νββ decay [5,[28][29][30][31][32] or a comparison of results for 0νββ decay in two or more isotopes [33][34][35][36]. These two approaches are studied in this paper.…”

Double beta decay and neutrino mass models

“…For example, measurements of the angular correlation between the two electrons from 0νββ [10,24,25] or measuring double beta plus decays [26] offer the possibility to identify the Lorentz structure (equivalently the chiralities of the emitted electrons) of the LNV processes. However, realistically the SuperNEMO proposal [25] could only accumulate the necessary statistics to identify the Lorentz structure, if the half-live of 82 Se is below 10 26 ys, while there yet exists no experimental proposal with a sufficient sensitivity for 0νβ + /EC decays to make use of the ideas discussed in [26].…”