Evidence Increasingly Rules Out Light Sterile Neutrino Hypothesis

The Standard Model of Particle Physics includes three flavors of neutrinos - electron neutrinos, muon neutrinos and tau neutrinos, so named because particle physicists weren't feeling particularly creative when they were hypothesized (and ultimately discovered).

There needs to be a neutrino counterpart to each charged lepton to preserve the approximate lepton flavor conservation symmetry of the theory (neutrino oscillation prevents it from being a perfectly conserved symmetry), and weak force boson decays have long confirmed that there are three flavors of weakly interacting neutrinos, so three flavors of neutrinos are necessary to make electroweak theory conform to observation.

Anomalies in reactor neutrino data had suggested the possibility of a fourth light neutrino that oscillates with the three ordinary neutrinos, but do not interact via the weak force.

The cosmologically measured constant Neff (effective N) for the number of neutrino flavors increasingly disfavors a fourth type of neutrino that the existing three flavors oscillate with (but which does not interact via the weak force). So do new cosmological data bounds on the sum of the mass of all neutrino flavors because the difference between the maximum value of the sum of the masses of the three neutrino flavors and the minimum value derived from the differences in mass between the three primary neutrino mass states is increasingly small, placing an upper bound on the mass of any fourth sterile neutrino.

(Note, however, that the cosmology purposes, a neutrino with a mass far in excess of 1 eV/c^2 such as a sterile neutrino with a mass on the order of a keV, which has been proposed as a dark matter candidate, is outside the cosmology definition of a neutrino. The cosmology definition is largely synonymous with the definition of "hot dark matter", rather than using the usual Standard Model definition.)

This year's Neutrino 2016 Conference has produced three papers all documenting new experimental findings the strongly disfavor the kind of sterile neutrino that is light enough and mixed enough with the other three neutrino flavors to explain the apparent reactor neutrino anomalies that had prompted the light sterile neutrino hypothesis. One of the papers setting for the new experimental limits is based on data from the Daya Bay experiment, one is based on MINOS data, and one combines data from the Daya Bay, MINOS and Bugey-3 experiments to obtain a global exclusion based upon the latest data.

Taken together, the cosmology and earth based experimental exclusions provide a strong and robust exclusion of the light sterile neutrino hypothesis in all circumstances in which any other evidence might have suggested it in the first place (and the reactor anomaly itself have also grown less acute as more data and analysis have examined it).

Also, the titles and abstracts of the papers on neutrinoless double beta decay this year make clear that nobody has credibly observed neutrinoless double beta decay yet experimentally, so the experimental minimum bound on the potential mean time frame of neutrinoless double beta decay continues to get incrementally longer from the last time this data was reported.  No instances of lepton number violation have been observed in any other context either.