Double Beta Decay

TL;DR

This paper examines neutrinoless double beta decay as a probe of the Majorana nature of neutrinos and the absolute mass scale. It presents the theoretical framework linking the decay rate to the effective Majorana mass $\langle m_\nu \rangle$, the nuclear matrix elements $M_{GT}^{0ν}$ and $M_F^{0ν}$, and the phase-space factor $G^{0ν}$, with the rate proportional to $\langle m_ν \rangle^2$. It reviews experimental approaches, past results, and background challenges, highlighting the role of background suppression, energy resolution, and the use of source-as-detector detectology, as well as the two main theoretical methods for matrix elements, QRPA and NSM. The article surveys major future projects (CUORE, EXO, GENIUS, Majorana, MOON) and related proposals, emphasizing the goal of reaching sensitivities around $\langle m_\nu \rangle \lesssim 50$ meV and the critical importance of reducing nuclear-structure uncertainties for interpreting potential signals. Collectively, the work underscores the potential for groundbreaking insights into neutrino mass and lepton-number violation, contingent on achieving large isotopic masses and robust control of backgrounds.

Abstract

The motivation, present status, and future plans of the search for the neutrinoless double beta decay are reviewed. It is argued that, motivated by the recent observations of neutrino oscillations, there is a reasonable hope that neutrinoless double beta decay corresponding to the neutrino mass scale suggested by oscillations, <m_ν> of about 50 meV, actually exists. The challenges to achieve the sensitivity corresponding to this mass scale, and plans to overcome them, are described.

Figures (3)

• Figure 1: Illustration of the spectra of the sum of the electron kinetic energies K$_e$ (Q is the endpoint) for the $\beta\beta(2\nu)$ normalized to 1 (dotted curve) and $\beta\beta(0\nu)$ decays (solid curve). The $\beta\beta(0\nu)$ spectrum is normalized to $10^{-2}$ ($10^{-6}$ in the figure inset). All spectra are convolved with an energy resolution of 5%, representative of several experiments. However, some experiments, notably Ge, have a much better energy resolution.
• Figure 2: "Moore's law" of $\beta\beta(0\nu)$ decay: the limit of the effective neutrino mass vs. time. The corresponding experiments are denoted by the symbol for the initial nucleus. The uncertainty in the nuclear matrix elements is not included in this illustration. The gray band near the bottom indicates the neutrino mass scale $\sqrt{\Delta m_{atm}^2}\ $ .
• Figure 3: Effective mass $\langle m_{\nu} \rangle$ as a function of the smallest neutrino mass $m_{min}$. The left panel is for the normal mass hierarchy, as indicated in the insert (not to scale), and the right panel is for the inverted hierarchy. Both panels are evaluated for the LMA solar solution with $\Delta m^2_{atm} = 2.4 \times 10^{-3}$ eV$^2$, $\Delta m^2_{sol} = 4.5 \times 10^{-5}$ eV$^2$, and $|U_{e2}|^2 = 0.3$. The full lines show $\langle m_{\nu} \rangle$$_{max}$ and $\langle m_{\nu} \rangle$$_{min}$, defined in Equation \ref{['eq:minmax']}, for $U_{e3} = 0$ and the dashed lines use the maximum value $|U_{e3}|^2 = 0.025$ allowed by the Chooz and Palo Verde reactor experiments Chooz99Palo01.