The search for neutrinoless double beta decay

TL;DR

This review dissects the physics case for neutrinoless double beta decay and surveys both the theoretical framework for Majorana neutrinos and the experimental landscape. It links the effective Majorana mass m_{ββ} to decay rates via phase-space factors and NMEs, and analyzes how various nuclear-model approaches shape the interpretation of results. The authors compare current limits and forecast the sensitivities of next-generation, multi-ton experiments across isotopes such as ^{76}Ge, ^{82}Se, ^{130}Te, ^{136}Xe, and ^{150}Nd, highlighting the crucial roles of background suppression, energy resolution, and detector efficiency. Ultimately, observing ββ0ν would reveal lepton-number violation, establish neutrinos as Majorana particles, and offer insight into the absolute neutrino mass scale and the mechanism behind neutrino masses, with significant implications for particle physics and cosmology.

Abstract

In the last two decades the search for neutrinoless double beta decay has evolved into one of the highest priorities for understanding neutrinos and the origin of mass. The main reason for this paradigm shift has been the discovery of neutrino oscillations, which clearly established the existence of massive neutrinos. An additional motivation for conducting such searches comes from the existence of an unconfirmed, but not refuted, claim of evidence for neutrinoless double decay in $^{76}\text{Ge}$. As a consequence, a new generation of experiments, employing different detection techniques and $ββ$ isotopes, is being actively promoted by experimental groups across the world. In addition, nuclear theorists are making remarkable progress in the calculation of the neutrinoless double beta decay nuclear matrix elements, thus eliminating a substantial part of the theoretical uncertainties affecting the particle physics interpretation of this process. In this report, we review the main aspects of the double beta decay process and some of the most relevant experiments. The picture that emerges is one where searching for neutrinoless double beta decay is recognized to have both far-reaching theoretical implications and promising prospects for experimental observation in the near future.

Figures (32)

• Figure 1: Knowledge on neutrino masses and mixings from neutrino oscillation experiments. Panels (a) and (b) show the normal and inverted mass orderings, respectively. Neutrino masses increase from bottom to top. The electron, muon and tau flavor content of each neutrino mass eigenstate is shown via the red, green and blue fractions, respectively.
• Figure 2: Constraints on the lightest neutrino mass $m_{\rm light}$ coming from a) cosmological and b) $\beta$ decay experiments. The red and green bands correspond to the normal and inverted orderings, respectively. The $m_{{\rm cosmo}}$ upper bound in panel (a) is from Komatsu:2010fb, and translates into a $m_{{\rm light}}$ upper limit shown via the vertical band in the same panel. The cosmological constraint on $m_{{\rm light}}$ is also shown in panel (b), together with the upper limit on $m_{\beta}$ from tritium $\beta$ decay experiments Nakamura:2010zzi.
• Figure 3: The difference between Dirac (left) and Majorana (right) massive neutrinos in a scattering experiment. See text for details. Adapted from Parke:2011zz.
• Figure 4: Hierarchical structure of fermion masses. Only upper bounds for neutrino masses exist. The figure assumes a normal ordering for neutrino masses. Values taken from Nakamura:2010zzi.
• Figure 5: Feynman diagrams that contribute to the lepton number asymmetry through the decays of the heavy Majorana neutrino $N_1$ into the Higgs $\phi$ plus leptons $l_{\alpha}$. The asymmetry is generated via the interference of the tree-level diagram (a) with the one-loop vertex correction (b) and the self-energy (c) diagrams (adapted from Chen:2007fv).