Neutrinos and Dark Matter

We now know from observations of neutrino oscillations in reactor, accelerator, atmospheric, and solar neutrino based experiments that neutrinos have non-zero masses and that the flavor states associated with weak interactions (νe, νμ, and ντ) are superpositions of mass states (ν133). Three-family neutrino mass parameters (δ m2, Δ m2) and mixing angles (θ12, θ23, θ13) are sufficient to describe the established neutrino oscillation data, although unconfirmed experimental results hint that additional flavors of sterile neutrinos might exist. Global analysis of these neutrino oscillation data provide a high degree of consistency and stability in the parameter determination.

A series of fundamental questions remain to be addressed about neutrinos:
1) Are neutrinos Dirac or Majorana particles?
2) What are the absolute masses of neutrinos and what can we learn from the hierarchy of particle masses?
3) Is CP violated in the lepton sector and might neutrinos offer an explanation of the observed matter to antimatter asymmetry?
4) How have neutrinos shaped the evolution of the universe?
5) What is the value of θ13, and can we discern an underlying symmetry or mechanism that gives rise to the observed neutrino mixing matrix values?
6)Might additional flavors of sterile neutrinos exist?
The answers to these questions will shape a new Standard Model of fundamental interactions and possibly open a window to new neutrino states and interactions, while also impacting our understanding of astrophysics and cosmology.

Cross-sectional view of the MAJORANA DEMONSTRATOR, with one cryostat inserted and another being positioned for insertion into the shield. Cryostats are mounted on moveable transporters allowing assembly and testing before installation into the shield. The cryostat vacuum system is visible for the cryostat on the right. The graded shield consists of electroformed copper, commercial copper, lead, a radon exclusion volume, an active muon scintillator veto, and a polyethylene neutron moderator. For scale, the inner Cu volume where the cryostats are inserted is 20" high, 20" wide, and 30" in length.

TUNL Neutrino Research Program

The TUNL neutrino research program currently focuses on the first four of the above items, with experiments searching for neutrinoless double beta decay and directly probing absolute neutrino mass. We are also exploiting the unique capabilities of our 76Ge point-contact detectors to search for dark matter via observing nuclear recoils at very low energies (<1 keV). Finally, we are working on a number of ancillary nuclear physics measurements that will provide important input or constraints for neutrino and dark matter experiments.

TUNL scientists, students, and staff are playing leading roles in two next-generation neutrino experiments: the MAJORANA Collaboration's activities to demonstrate the feasibility of a tonne scale 76Ge based 0νββ-decay experiment, and in the KArlsruhe TRItium Neutrino experiment (KATRIN), a tritium β-decay experiment with a sensitivity to sub-eV neutrino masses.