Lepton Number Violation (Majorana Demonstrator)
There are three very important open questions in neutrino physics that can best be addressed by next generation zero-neutrino double-beta (0νββ) decay experiments. Are neutrinos Majorana particles that differ from antineutrinos only by helicity? What is their mass-scale? Is lepton number conservation violated? Measurements of atmospheric, solar, and reactor neutrino oscillations have revealed scenarios in which the effective Majorana mass of the electron neutrino could be larger than 5 meV. Recent developments in Ge-detector technology now make the search for zero-neutrino double-beta decay of 76Ge at this scale feasible.
A complete understanding of the neutrino mass matrix depends on four types of data: neutrino oscillations, direct kinematical measurements, cosmological observations, and zero-neutrino double-beta. The results of atmospheric, solar, and reactor neutrino oscillation experiments have provided evidence for neutrino mass, and have determined the relative splitting of the mass eigenstates. These experiments show that at least one neutrino has a mass greater than 50 meV. The absolute scale can only be obtained from direct mass measurements (such as 3H end-point measurements), cosmological observations, or by zero-neutrino double-beta in the case that the neutrino is a Majorana particle (its own antiparticle). Not only is zero-neutrino double-beta the only practical method to uncover the Majorana nature of neutrinos, it is perhaps the only practical method to reach an absolute mass scale sensitivity of <50 me V. In addition, zero-neutrino double-beta experiments complement long-baseline neutrino oscillation experiments in establishing the hierarchy of the neutrino mass eigenstates.
The ultimate objective of the Majorana collaboration is to study neutrinoless double-beta decay with an effective Majorana-neutrino mass sensitivity below 50 meV to characterize the Majorana nature of the neutrino, the neutrino mass spectrum, and the absolute mass scale. An experimental study of the neutrino mass scale implied by neutrino oscillation results is now technically within our grasp. The GERDA and Majorana collaborations intend to eventually come together to construct a tonne-scale zero-neutrino double-beta decay experiment. PNNL was one of the initiators of the Majorana Experiment, providing our expertise in low background detection technologies.