Neutrinoless double beta decay: 2015 review

TL;DR

This 2015 review analyzes neutrinoless double beta decay ($0\nu\beta\beta$) as a probe of lepton-number violation and the Majorana nature of neutrinos. It foregrounds the light Majorana-neutrino exchange as the most plausible mechanism, while surveying alternative11 mechanisms via higher-dimensional operators, heavy-neutrino exchange, and RH currents. It integrates particle-physics basics (neutrino masses, oscillations, and cosmology) with the nuclear-physics challenges (phase-space factors and NMEs) and provides a comprehensive assessment of current and planned experiments, emphasizing the dominant theoretical uncertainty from $g_A$ quenching. The review highlights the powerful interplay with cosmology, showing how bounds on the sum of neutrino masses $\Sigma$ constrain $m_{\beta\beta}$ and guide future experimental sensitivities toward the inverted hierarchy region, with cosmology potentially dictating the scale and reach of upcoming detectors. Overall, it underscores the need for cross-disciplinary advances in nuclear theory, astrophysical data, and large-scale detector technology to realize the potential of $0\nu\beta\beta$ to illuminate fundamental neutrino properties and the origin of matter in the universe.

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.

Figures (22)

• Figure 1: Diagram of the $0\nu \beta \beta$ process due to the exchange of massive Majorana neutrinos, here denoted generically by $\nu_{\hbox{\tiny{M}}}$.
• Figure 2: Massive fields in their rest frames. The arrows show the possible directions of the spin. (Left) The 4 states of Dirac massive field. The signs indicate the charge that distinguishes particles and antiparticles, e. g. the electric charge of an electron. (Right) The 2 states of Majorana massive field. The symbol "zero" indicates the absence of any $U(1)$ charge: particles and antiparticles coincide.
• Figure 3: The chiral nature of weak interactions allows us to define what is a neutrino and what it an antineutrino in the ultra-relativistic limit, when chirality coincides with helicity and the value of the mass plays only a minor role.
• Figure 4: Helicity of the 15 massless matter particles contained in each family of the SM (see Table \ref{['tab:fields']}). The arrow gives the direction of the momentum.
• Figure 5: Diagram representing the contribution of the "black box" operator to the Majorana mass. Figure from Ref. Duerr:2011zd.