Combined Reactor Data Basically Rules Out Sterile Neutrinos

One of the ways we study neutrinos, and in particular, their oscillations between neutrino masses and flavors described in the Standard Model by the PMNS matrix, is to compare the mix of neutrino types produced at a reactor source with the mix of neutrino types observed at a distant neutrino flavor detector.

Some of these experiments have shown results that individually seem inconsistent with the Standard Model scenario in which there are three neutrinos and three neutrino mass eigenstates that oscillate between each other. 

W and Z boson decays strongly confirm that only there are only three neutrino flavors with masses of 45 GeV (i.e. 1,000,000,000 eV) or less that interact via the weak force that also drives nuclear beta decay (constraints on the largest of the neutrino masses from astronomy observations are on the order of 0.1 eV at most for the heaviest neutrino mass, with direct observations limiting these masses to on the order of 1 eV). But, it is harder to rule out the possibility that there might be a fourth "sterile neutrino" (or possibly additional sterile neutrinos as well) that oscillates with other flavors of neutrinos in an extended PMNS matrix, but do not interact via electromagnetism, the strong force, or the weak force.[1]

Individually, reactor neutrino oscillation measurements show deviations from the expected values that can be interpreted as a eV mass scale sterile neutrino that oscillates via with the three active neutrinos, but does not interact via the weak force. 

But, the experimental evidence from various reactor experiments for a sterile neutrino reactor anomaly compared across all of the experiments simultaneously is mutually inconsistent, essentially ruling out the existence of sterile neutrinos at a statistically significant level over essentially all of the plausible parameter space for sterile neutrinos.

These results compliment astronomy measurements interpreted in light cosmology models regarding of the number of effective neutrino flavors (Neff), which also strongly favor a three flavor possibility over a four plus flavor possibility.

Previous papers have suggested that apparent sterile neutrino reactor anomalies may actually represent a failure to model the neutrino production of nuclear reactor sources with varied proportions of different nuclear fuels, or other methodological errors.

The new paper and its abstract are as follows:

Searches for electron antineutrino, muon neutrino, and muon antineutrino disappearance driven by sterile neutrino mixing have been carried out by the Daya Bay and MINOS+ collaborations. This Letter presents the combined results of these searches, along with exclusion results from the Bugey-3 reactor experiment, framed in a minimally extended four-neutrino scenario. Significantly improved constraints on the θμe mixing angle are derived that constitute the most stringent limits to date over five orders of magnitude in the sterile mass-squared splitting Δm241, excluding the 90% C.L. sterile-neutrino parameter space allowed by the LSND and MiniBooNE observations at 90% CLs for Δm241<5eV2.Furthermore, the LSND and MiniBooNE 99% C.L. allowed regions are excluded at 99% CLs for Δm241 < 1.2 eV2.

Daya Bay, MINOS+ Collaborations, "Improved Constraints on Sterile Neutrino Mixing from Disappearance Searches in the MINOS, MINOS+, Daya Bay, and Bugey-3 Experiments" arXiv 2002.00301 (February 2, 2020).

[1] Fundamental particles with spin 1/2 are called "leptons" and leptons with no electromagnetic charge are called neutrinos. They were originally hypothesized to exist to maintain conservation of mass-energy, linear and angular momentum and lepton number in nuclear beta decay interactions.

Charged fundamental particles of a given flavor with spin 1/2 all have four subtypes (left parity particles, right parity particles, left partity antiparticles, and right parity antiparticles) (with three color variants of each of these four subtypes for quarks), that are otherwise identical.

But, neutrinos in the Standard Model come in only two types (left parity particles and right parity antiparticles).

This is because, in the Standard Model, the weak force interacts only with left particle particles and right parity antiparticles. The parity counterparts of these particles do not interact via the weak force. Further, leptons, in general do not interact via the strong force of the Standard Model, and neutrinos have no electromagnetic charge.

Thus, right parity neutrinos and left parity antineutrinos, if they existed in a beyond the Standard Model extension, would not interact via any of the three Standard Model forces. So, another name for sterile neutrinos is "right handed neutrinos", even though, in theory, a neutrino could lack all three Standard Model force interactions for reasons than their parity.

The interaction with the Higgs field that gives rise to fundamental particle mass in the equations of the Standard Model presumes the existence of a particle that has both left parity and right parity versions of every fundamental particle and antiparticle. But, these equations break down in the case of neutrinos which have only one parity of particle and one partity of antiparticle for each of the three active flavors.

For this reason, for a long time, the Standard Model assumed that neutrinos, which are by far the lightest fundamental fermions, had no mass rest mass at all, and hence didn't need to derive mass from the Higgs field, something that was experimentally disproven a couple of decades later.

As a result neutrino masses are a little corner of the modern Standard Model that isn't completely worked out yet, and there are competing explanations for their masses.

As I explained in comments to another recent post at this blog, I don't think any of the leading explanations of neutrinos mass (either a see-saw mechanism with right handed neutrinos, or "Majorana mass" associated with a particle being its own anti-particle) are very plausible, although I don't have another alternative to offer.