Long-lived Sterile Neutrinos at Belle II in Effective Field Theory

TL;DR

This paper investigates the potential of Belle II to detect long-lived sterile neutrinos through displaced-vertex signatures, exploring both standard mixing and higher-dimensional operators within an effective field theory framework.

Contribution

It introduces a comprehensive EFT approach to sterile neutrino searches at Belle II, including Monte Carlo simulations to estimate sensitivities to non-renormalizable operators.

Findings

Belle II can probe dimension-six operators up to a few TeV.

Sterile neutrinos can be detected via displaced vertices.

The study extends the search strategies beyond standard mixing.

Abstract

Large numbers of $\tau$ leptons are produced at Belle II. These could potentially decay into sterile neutrinos that, for the mass range under consideration, are typically long-lived, leading to displaced-vertex signatures. Here, we study a displaced-vertex search sterile-neutrino-extended Standard Model Effective Field Theory. The production and decay of the sterile neutrinos can be realized via either the standard active-sterile neutrino mixing or higher-dimensional operators in the effective Lagrangian. We perform Monte-Carlo simulations to estimate the Belle II sensitivities to such interactions. We find that Belle II can probe non-renormalizable dimension-six operators involving a single sterile neutrino up to a few TeV in the new-physics scale.