Elsevier

Physics Reports

Volume 884, 13 November 2020, Pages 1-59
Physics Reports

Where are we with light sterile neutrinos?

https://doi.org/10.1016/j.physrep.2020.08.005Get rights and content

Abstract

We review the status of searches for sterile neutrinos in the 1eV range, with an emphasis on the latest results from short baseline oscillation experiments and how they fit within sterile neutrino oscillation models. We present global fit results to a three-active-flavor plus one-sterile-flavor model (3+1), where we find an improvement of Δχ2=35 for 3 additional parameters compared to a model with no sterile neutrino. This is a 5σ improvement, indicating that an effect that is like that of a sterile neutrino is highly preferred by the data. However we note that separate fits to the appearance and disappearance oscillation data sets within a 3+1 model do not show the expected overlapping allowed regions in parameter space. This “tension” leads us to explore two options: 3+2, where a second additional mass state is introduced, and a 3+1+decay model, where the ν4 state can decay to invisible particles. The 3+1+decay model, which is also motivated by improving compatibility with cosmological observations, yields the larger improvement, with a Δχ2=8 for 1 additional parameter beyond the 3+1 model, which is a 2.6σ improvement. Moreover the tension between appearance and disappearance experiments is reduced compared to 3+1, although disagreement remains. In these studies, we use a frequentist approach and also a Bayesian method of finding credible regions.

With respect to this tension, we review possible problems with the global fitting method. We note multiple issues, including problems with reproducing the experimental results, especially in the case of experiments that do not provide adequate data releases. We discuss an unexpected 5 MeV excess, observed in the reactor flux energy spectrum, that may be affecting the oscillation interpretation of the short baseline reactor data. We emphasize the care that must be taken in mapping to the true neutrino energy in the case of oscillation experiments that are subject to multiple interaction modes and nuclear effects. We point to problems with the “Parameter-Goodness-of-Fit test” that is used to quantify the tension. Lastly, we point out that analyses presenting limits often receive less scrutiny that signals.

While we provide a snapshot of the status of sterile neutrino searches today and global fits to their interpretation, we emphasize that this is a fast-moving field. We briefly review experiments that are expected to report new data in the immediate future. Lastly, we consider the 5-year horizon, where we propose that decay-at-rest neutrino sources are the best method of finally resolving the confusing situation.

Introduction

From the beginning, neutrino physics has been propelled forward by pursuit of anomalies. Of these, some eventually developed into decisive signals, laying the ground work for today’s “neutrino Standard Model” (νSM). Others were disproven, and forced us to improve our understanding of neutrino models, sources, and detectors in the process. In keeping with this cycle, anomalies have been observed in short-baseline (SBL) oscillation experiments since the 1990s; see [1], [2], [3] for other reviews on this topic. These potentially point to the existence of a new kind of neutrino, called a sterile neutrino, although other experiments have substantially limited exotic neutrino interpretations. Resolving the question of whether these results point to new physics is a priority of our field. However, in the past year alone, the confusion has only mounted.

In this review, we consider the present status of the short baseline anomalies and their interpretations. We explain the motivation for, and phenomenology of, sterile neutrinos. We provide updated global fits to relevant data sets in the simplest single sterile neutrino model along with discussions of their frequentist and Bayesian interpretations. Since the global fits point to data discrepancies with this simple model, we also consider more complex explanations. Finally, we discuss how future measurements could impact our understanding.

Section snippets

The road to oscillations is paved with interesting anomalies

For the sake of this discussion, we will define an “anomalous signal” as a 2σ effect with no clear Standard Model (SM) explanation. We freely admit that this is an arbitrary line that reflects the personal taste of the authors on the point where a signal reaches a significance making it worthy of further exploration. Using this definition, several anomalies appear in short baseline νμνe appearance and νeνe disappearance oscillation experiments.

However, before leaping in to these relatively

Two, three, four, and more

The resolution of the anomalies described above came about from introducing neutrino mass and mixing into the picture, which leads directly to an effect called vacuum neutrino oscillations. Before considering the νSM, which involves three neutrino flavors, it is instructive to introduce the phenomenology of neutrino oscillations in a two-neutrino picture. This picture will also be useful as we consider the new set of anomalies that lead to the potential introduction of sterile neutrinos as

Design of short baseline experiments

Accessing an oscillation signal region requires selection of neutrino sources that can produce the flavor of interest, and a detector which can observe such a flavor. The designer must also select the appropriate LE for the parameter space of interest, and this additionally influences the choice of source and detector. Large distance-of-travel requires intense sources and large detectors. The selected energy range affects the choice of source. This usually leads to a limited range of high-rate

Techniques of global fits

The experimental results discussed previously paint a disparate picture of the sterile neutrino landscape. If we are to make sense of the available data, they must be combined into a single analysis that considers all results simultaneously: a global fit. Given a model hypothesis, a global fit computes the likelihood for each experiment and combines them into a global likelihood.

Where experimental results agree, the global likelihood will be reinforced. This reinforcement reduces the

Models and global fit results

This section describes the results of global fits to the short baseline data. We begin with a 3+1 model and show that the global fits have a strong preference for the 3+1 solution compared to the three-neutrino solution. However, we show that this model lacks internal consistency, as taking arbitrary subsets of the global data produce incompatible results. This underlying disagreement opens the question of whether the 3+1 model is over-simplified, or if the anomalies are due to some other

What can possibly go wrong?

While global fits can provide general guidance, there are a number of issues that can bias the results. In this section we examine some of the features of the data that may contribute uncertainty to the global fit results. Ideally these uncertainties would be quantified, but at present it is not clear how this may be performed. Therefore, we simply present a qualitative discussion of things that can, possibly, go wrong.

Results beyond vacuum oscillation experiments

Our global fits focus on results from accelerator and reactor sources that can be interpreted using vacuum oscillations. However, there are other methods of sterile neutrino searches which use signatures beyond vacuum-oscillations. In this section, we briefly review other approaches using atmospheric, solar, and astrophysical neutrinos. We also touch on the ongoing controversy concerning sterile neutrinos and cosmology.

The immediate future for short-baseline results

Returning to our focus on sterile neutrino searches at man-made sources, we emphasize that this is an exciting and fast-growing field. Within the next two years, a number of the experiments already included in the global fits will provide important updates. In this section, we review experiments that will provide additional results within the next two years, beyond the experiments already included.

The next generation: What will resolve the sterile neutrino picture?

The past approach for addressing sterile neutrino anomalies has been to develop new experiments that are “good enough” under the best of conditions to provide some new information. This strategy will likely continue to result in leaving the field in a confusing situation since most of the new experiments cannot provide decisive, highly significant results. As a comparison, the sterile neutrino situation now is similar to the three-neutrino oscillation results available in the late 1980s​ and

Conclusions

In conclusion, this paper has provided a snapshot of where we are at in exploring the question of the existence of light sterile neutrinos, especially through accelerator and reactor experiments. The picture is far from clear.

Anomalies have been observed in a set of short-baseline experiments. Introducing an additional, mostly sterile mass state to explain this provides an improvement of >5σ, which is highly improbable as an accidental improvement. We find that adding additional sterile

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

AD, CA and JMC are supported by NSF grant PHY-1801996. MHS is supported by NSF grant PHY-1707971. GHC is supported by Institute for Data, Systems, and Society at MIT . We thank Roger Barlow, Paul Grannis, Adrien Hourlier, Patrick Huber, Keng Lin, Bryce Littlejohn, William Louis, Pedro Machado, Sergio Palomares-Ruiz, Jordi Salvado, Robert Shrock, and Lindley Winslow for valuable discussions. We thank MicroBooNE for the approved event display appearing in Fig. 32. We thank the STEREO experiment

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