Neutrino Physics

The recent observation of neutrino flavor oscillations has provided the first smoking gun evidence for physics beyond the Standard Model (SM) and put neutrinos at the forefront of particle physics, nuclear physics, and cosmology and astrophysics. While we now know that neutrinos have non-zero mass and know a great deal about the mass-squared differences and flavor mixing angles, there remain a number of important, open questions to be addressed theoretically and experimentally. For example, we would like to know whether the neutrino is its own antiparticle (a so-called Majorana particle), or has a distinct antiparticle like the other fermions of the SM. We would also like to know what the small, but non-zero neutrino mass implies for its other properties, such as the strength of its interactions with photons or the character of its involvement in weak decays. An question is whether neutrino interactions violate CP-symmetry and - if so - whether this CP-violation is strong enough to explain the baryon asymmetry through the leptogenesis mechanism (see the Baryogenesis page).

I have been interested in various aspects of these neutrino-related questions. Recently, for example, my collaborators and I studied the interpretation of neutrinoless double b-decay (0nbb) of heavy nuclei. If a non-zero rate for this process - which violates lepton number conservation -- is observed, it would provide incontrovertible evidence that the neutrino is its own antiparticle. In addition, one would like to use such a signal to determine the absolute scale of neutrino mass - something we cannot determine from neutrino oscillation experiments. In order to do so, however, one needs to know that the 0nbb process is dominated by the exchange of light, Majorana neutrinos and not by the exchange of heavy Majorana particles, such as the neutralinos of supersymmetry, that have lepton number violating interactions with the fermions of the SM. We have developed an effective field theory framework for analyzing these possible heavy particle contributions to the 0nbb rate. We have also analyzed the information that lepton flavor violating processes -- such as m to e conversion in nuclei - might provide about the size of these possible heavy particle contributions to the 0nbb rate.

Although we do not know the absolute neutrino mass scale, we do have upper bounds on the non-zero neutrino mass from ordinary tritium b-decay experiments and from studies of the cosmic microwave background. The small magnitude of these upper bounds has important implications for other neutrino properties and interactions. For example, we recently derived model-independent upper bounds on the magnetic moments of Dirac neutrinos as implied by the neutrino mass bounds and are currently involved in analyzing the corresponding implications for weak decays and scattering processes that involve neutrinos.