Non-standard interactions versus non-unitary lepton flavor mixing at a neutrino factory

TL;DR

This paper compares non-standard interactions and non-unitarity effects in neutrino oscillations at a neutrino factory, highlighting their similarities, differences, and how they might be distinguished experimentally.

Contribution

It demonstrates that NSIs and NU can produce similar effects but can be distinguished through specific measurements, especially in the $u- au$ sector, and discusses the role of near detectors.

Findings

NSIs and NU lead to similar phenomenological effects at neutrino factories.

Differences between NSIs and NU can be identified in the u- au sector for large effects.

Near detectors can help distinguish between source and matter effects, breaking correlations.

Abstract

The impact of heavy mediators on neutrino oscillations is typically described by non-standard four-fermion interactions (NSIs) or non-unitarity (NU). We focus on leptonic dimension-six effective operators which do not produce charged lepton flavor violation. These operators lead to particular correlations among neutrino production, propagation, and detection non-standard effects. We point out that these NSIs and NU phenomenologically lead, in fact, to very similar effects for a neutrino factory, for completely different fundamental reasons. We discuss how the parameters and probabilities are related in this case, and compare the sensitivities. We demonstrate that the NSIs and NU can, in principle, be distinguished for large enough effects at the example of non-standard effects in the $\mu$-$\tau$-sector, which basically corresponds to differentiating between scalars and fermions as heavy mediators as leading order effect. However, we find that a near detector at superbeams could provide very synergistic information, since the correlation between source and matter NSIs is broken for hadronic neutrino production, while NU is a fundamental effect present at any experiment.