Where are we with light sterile neutrinos?
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” (SM). 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 2 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 appearance and 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 SM, 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 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 , 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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