Phenomenology of Sterile Neutrinos
Antonio Palazzo (right) discussed the sterile neutrino hypothesis on the basis of recent experimental results.
After 20 years we finally know that the three-flavour scheme is what we should take as our standard framework to interpret the data. Yet the nature of the neutrino, Dirac or Majorana, is still unknown. Today we have a two sigma preference for a normal mass ordering.
Some extensions of the Standard Model predict sterile neutrinos. It is a very economical extension. They have thus been investigated at several scales, in collider experiments, as dark matter candidates, and in neutrino experiments at short and long baseline oscillation experiments, as well as in reactor and solar neutrino studies.
There are anomalies in experimental results. A unexplained excess of electron neutrino appearance in muon neutrino beams. THis is called “LSND anomaly”, confirmed by MiniBooNE. If we explain this in terms of oscillations, we need to extend the three-flavor paradigm.
A second anomaly “Gallium anomaly” describes the unexplained electron neutrino disappearance. Reactors give some deficit of flux, and indicate a bump at 5 MeV in the spectrum which is not predicted by models; fortunately, the extraction of theta_13 is not affected by this feature, being derived from a comparison of near and far detector fluxes. One can have two different perspectives in interpreting the experimental input: limits, or a delta chisquared calculation which can indicate the presence of sterile neutrinos.
In muon neutrino disappearance there is no anomaly, and this is a problem for the sterile neutrino hypothesis, because in all models including steriles, there is a general relation connecting the amplitude of the appearance and the product of the disappearances in electron and muon channels (Arxiv:1107.1452, Giunti and Laveder).
It is very interesting that NEOS selects a parameter space region for oscillations to sterile neutrinos which is within the best fit region of the other experiments. This is shown in the figure above, taken from Arxiv:1703.00860.
Palazzo then discussed how one could enlarge the 3-flavor scheme. One gets six mixing angles in place of three in an extended 3+1 matrix, and this brings in additional sources of CP violation. There are visible effects in the analysis, so in the 3+1 scheme the long-baseline constraints change significantly.