Non-Unitarity, sterile neutrinos, and Non-Standard neutrino Interactions

TL;DR

This paper compares sterile neutrinos and non-unitarity effects in neutrino oscillations, clarifies their parametrizations, and assesses their impact on current and future experiments like DUNE.

Contribution

It unifies the descriptions of sterile neutrinos and non-unitarity effects within the NSI framework and evaluates their experimental significance.

Findings

Sterile neutrino effects average out at the near detector, mimicking non-unitarity at the far detector.

Non-unitarity effects are too constrained to influence current or upcoming oscillation experiments.

Sterile neutrinos could significantly affect long baseline experiments like DUNE.

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.