Virtual Majorana Neutrinos and the Minimum Neutrino Mass Scale in Neutrinoless Double-Beta Decay

Abstract

Virtual Majorana neutrinos are indispensable for neutrinoless double-beta (0$\nu\beta\beta$) decay. In this study, we demonstrate that the overlap of the virtual Majorana neutrino wavefunction, predominantly composed of a right-handed antineutrino component with a strongly suppressed left-handed component (with amplitude proportional to the effective Majorana neutrino mass, $|m_{\beta\beta}|$, is crucial for triggering this decay process. This effective mass, derived from the minimum neutrino mass, offers valuable insights into the absolute neutrino mass scale. Using best-fit parameters from neutrino oscillation experiments, the minimum neutrino mass is determined from the sum of the three neutrino mass eigenstates, $\Sigma = m_1 + m_2 + m_3,$ which is represented by two narrow bands centered at approximately 0.06 eV/c$^2$ for the normal hierarchy (NH) and 0.102 eV/c$^2$ for the inverted hierarchy (IH). Under these constraints, the minimum neutrino mass is found to be 0.001186 eV/c$^2$ for NH and 0.002646 eV/c$^2$ for IH, thereby establishing a potential absolute neutrino mass scale for both scenarios. From these values, we calculate $|m_{\beta\beta}|$, which plays a central role in $0\nu\beta\beta$ decay. By combining $|m_{\beta\beta}|$ with decay phase-space factors, nuclear matrix elements, and the absorption probability of the virtual Majorana neutrino, we estimate the $0\nu\beta\beta$ half-life for key isotopes, namely, $^{76}$Ge, $^{130}$Te, and $^{136}$Xe, using two independent methods. The results are in good agreement, and we also discuss the uncertainties in the nuclear matrix elements that may affect these calculations.