Cosmology Data Almost Rules Out Even Exotic Sterile Neutrino Models

One way to explain the MiniBooNE anomaly (which may or may not be real) is that it is caused by a "sterile neutrino" which is a neutrino that oscillates with other active neutrino types but has no other Standard Model of particle physics interactions with other kinds of particles.

This is disfavored by astronomy measurements, including N_eff (the effective number of neutrino species). As noted in the MiniBooNE link: "[A]s of 2015, the constraint with Planck data and other data sets was [N_eff is equal to] 3.04 ± 0.18." N_eff equal to 3.046 in a case with the three Standard Model neutrinos and neutrinos with masses of 10 eV or more do not count in the calculation. A light (under 10 eV) sterile neutrino that oscillated with the active neutrinos would push N_eff to 4.05 or so, which is ruled out at the five plus sigma level. Unlike limits on the number of active neutrino types from W and Z boson decays, the strong restrictions from N_eff can apply to "right handed" neutrinos that oscillate with other neutrinos, but do not interact via the weak force.

Cosmology constraints from N_eff measurements can be evaded with a "secret interaction" model, in which sterile neutrinos do not "freeze out" at the time when the astronomy features which are measured to determine N_eff come into being. But, new cosmology constraints disfavor even this model, leaving only even more Baroque forms of sterile neutrinos as the only way to accommodate the cosmology data.

Several long-standing anomalies from short-baseline neutrino oscillation experiments -- most recently corroborated by new data from MiniBooNE -- have led to the hypothesis that extra, 'sterile', neutrino species might exist. Models of this type face severe cosmological constraints, and several ideas have been proposed to avoid these constraints. Among the most widely discussed ones are models with so-called 'secret interactions' in the neutrino sector. In these models, sterile neutrinos are hypothesized to couple to a new interaction, which dynamically suppresses their production in the early Universe through finite-temperature effects. Recently, it has been argued that the original calculations demonstrating the viability of this scenario need to be refined. Here, we update our earlier results from arXiv:1310.6337 [JCAP 1510 (2015) no.10, 011] accordingly. We confirm that much of the previously open parameter space for secret interactions is in fact ruled out by cosmological constraints on the sum of neutrino masses and on free-streaming of active neutrinos. We then discuss possible modifications of the vanilla scenario that would reconcile sterile neutrinos with cosmology.

Xiaoyong Chu, et al., "Sterile Neutrinos with Secret Interactions -- Cosmological Discord?" (June 27, 2018).

Normal v. Inverted Hierarchy and Absolute Neutrino Masses

Astronomy data can now credibly support a 0.091 eV upper limit on the sum of the three active neutrino masses at a 95% confidence level (i.e. 2 sigma). The "normal" neutrino mass hierarchy is now favored over the "inverted" neutrino mass hierarchy at the 3.5 sigma level by existing available data according to the linked review article.

A lower bound based upon neutrino oscillation experiments can be put on the sum of the three neutrino masses for each of the two scenarios. For a normal hierarchy, it is 0.0585 ± 0.00048 eV, for an inverted hierarchy it is 0.0986 ± 0.00085 eV.

Among other things this puts an upper bound the the lightest neutrino mass eigenvalue of less than 11 meV. There are also strong qualitative arguments for the lightest neutrino mass eigenvalue being non-zero, because massless particles behave differently than particles with any mass whatsoever, although the oscillation data alone does not compel this result. So, we really know the absolute neutrino masses of all three types to a precision of about +/- 0.0055 eV (with a strong bias towards the low end), and almost all of this error is perfectly correlated between the three neutrino masses. 

So, even though we don't have a direct measurement to confirm this fact, we actually have a pretty good idea regarding the absolute neutrino masses at this point, as well as their hierarchy.

Neutrinoless double beta decay experiments haven't quite done so yet, but we only need to improve their precision by a factor of about 30 to definitively determine if neutrino mass is Majorana or Dirac, which is an attainable goal that could be achieved in my lifetime and probably even a decade or so. As I've stated repeatedly before at this blog, my prediction is that the neutrino masses will turn out to be Dirac, contrary to the majority position in the theoretical physics community, for a variety of reasons.