Abstract:The simplest Standard Model extension to explain neutrino masses involves the addition of right-handed neutrinos. At some level, this extension will impact neutrino oscillation searches. In this work we explore the differences and similarities between the case in which these neutrinos are kinematically accessible (sterile neutrinos) or not (mixing matrix non-unitarity). We clarify apparent inconsistencies in the present literature when using different parametrizations to describe these effects and recast both limits in the popular neutrino non-standard interaction (NSI) formalism. We find that, in the limit in which sterile oscillations are averaged out at the near detector, their effects at the far detector coincide with non-unitarity at leading order, even in presence of a matter potential. We also summarize the present bounds existing in both limits and compare them with the expected sensitivities of near-future facilities taking the DUNE proposal as a benchmark. We conclude that non-unitarity effects are too constrained to impact present or near future neutrino oscillation facilities but that sterile neutrinos can play an important role at long baseline experiments. The role of the near detector is also discussed in detail.

Future high-precision neutrino interaction experiments are needed to extend the current program of GeV-scale neutrino interactions and should include:1. A feasibility study of a high-statistics hydrogen or deuterium scattering experiment to supplement the currently poorly known (anti)neutrino-nucleon cross sections.2. The need for (anti)neutrino Ar scattering data in the energy range relevant for the DUNE experiment.3. The possibility of muon-based neutrino beams providing extremely accurate knowledge of the neutrino flux and an intense electron neutrino beam.• Current and future long-and short-baseline neutrino oscillation programs should evaluate and articulate what additional neutrino-nucleus interaction data is required to meet their ambitious goals and support experiments that provide this data.In addition to these general challenges facing the community, there are more specific concerns for particular topics and interaction channels. These are summarized below in the form of observations, problem description or recommendations. For a deeper insight, the reader is encouraged to consult the subsequent sections of this paper.

In the presence of non-standard neutrino interactions (NSI), oscillation data are affected by a degeneracy which allows the solar mixing angle to be in the second octant (aka the dark side) and implies a sign flip of the atmospheric mass-squared difference. This leads to an ambiguity in the determination of the ordering of neutrino masses, one of the main goals of the current and future experimental neutrino program. We show that the recent observation of coherent neutrino-nucleus scattering by the COHERENT experiment, in combination with global oscillation data, excludes the NSI degeneracy at the 3.1σ (3.6σ) CL for NSI with up (down) quarks.The standard three-flavour oscillation scenario is supported by a large amount of data from neutrino oscillation experiments. The determination of oscillation parameters (see, e.g., Ref.[1]) is very robust, and for a broad range of new physics scenarios only small perturbations of the standard oscillation picture are allowed by data. There is, however, an exception to this statement: in the presence of non-standard neutrino interactions (NSI) [2-4] a degeneracy exists in oscillation data, leading to a qualitative change of the lepton mixing pattern. This was first observed in the context of solar neutrinos, where for suitable NSI the data can be explained by a mixing angle θ 12 in the second octant, the so-called LMA-Dark (LMA-D) [5] solution. This is in sharp contrast to the established standard MSW solution [2,6], which requires a mixing angle θ 12 in the first octant.The origin of the LMA-D solution is a degeneracy in oscillation probabilities due to a symmetry of the Hamiltonian describing neutrino evolution in the presence of NSI [7][8][9][10]. This degeneracy involves not only the octant of θ 12 but also a change in sign of the larger neutrino mass-squared difference, ∆m 2 31 , which is used to parameterize the type of neutrino mass ordering (normal versus inverted). Hence, the LMA-D degeneracy makes it impossible to determine the neutrino mass ordering by oscillation experiments [10], and therefore jeopardizes one of the main goals of the upcoming neutrino oscillation program. As discussed in Refs. [5,[10][11][12], non-oscillation data (such as that from neutrino scattering experiments) is needed to break this degeneracy.Recently, coherent neutrino-nucleus scattering has been observed for the first time by the COHERENT experiment [13], using neutrinos produced at the Spallation Neutron Source (SNS) in Oak Ridge National Laboratory. The observed interaction rate is in good agreement with the Standard Model (SM) prediction and can be used to constrain NSI. In this Letter we show that this result excludes the LMA-D solution at 3.1σ (3.6σ) CL for NSI with up (down) quarks when combined with oscillation data.NSI formalism and the LMA-D degeneracy. We consider the presence of neutral-current (NC) NSI in the form of dimension-six four-fermion operators, following the notation of Ref. [8]. Since we are interested in the contribution of the NSI to the effective potential of ne...

In presence of non-standard neutrino interactions the neutrino flavor evolution equation is affected by a degeneracy which leads to the so-called LMA-Dark solution. It requires a solar mixing angle in the second octant and implies an ambiguity in the neutrino mass ordering. Non-oscillation experiments are required to break this degeneracy. We perform a combined analysis of data from oscillation experiments with the neutrino scattering experiments CHARM and NuTeV. We find that the degeneracy can be lifted if the non-standard neutrino interactions take place with down quarks, but it remains for up quarks. However, CHARM and NuTeV constraints apply only if the new interactions take place through mediators not much lighter than the electroweak scale. For light mediators we consider the possibility to resolve the degeneracy by using data from future coherent neutrino-nucleus scattering experiments. We find that, for an experiment using a stoppedpion neutrino source, the LMA-Dark degeneracy will either be resolved, or the presence of new interactions in the neutrino sector will be established with high significance.

A variety of new physics models allows for neutrinos to up-scatter into heavier states. If the incident neutrino is energetic enough, the heavy neutrino may travel some distance before decaying. In this work, we consider the atmospheric neutrino flux as a source of such events. At IceCube, this would lead to a "double-bang" (DB) event topology, similar to what is predicted to occur for tau neutrinos at ultra-high energies. The DB event topology has an extremely low background rate from coincident atmospheric cascades, making this a distinctive signature of new physics. Our results indicate that IceCube should already be able to derive new competitive constraints on models with GeV-scale sterile neutrinos using existing data.